Silverstripe-framework: Run PHP 7.4 automation tests on all core modules

[chillu]

Acceptance Criteria

• Every module included in silverstripe-installer runs on PHP 7.4, individually
• Builds are added for both 3.x and 4.x versions. Note framework 3.x is already running these builds
• There is no composer constraint limiting the module use to a version less than PHP 7
• All builds are passing for 4.x.
• All failing builds in SS3 are split into new issues for tackling in the future.

Notes

• Framework already runs on PHP 7.4 after resolving a PHPUnit blocker
• Opened the same issue for PHP 7.2 and 7.3 at https://github.com/silverstripe/silverstripe-framework/issues/8864 a while ago
• Using php: 7.4snapshot in Travis now means we might need to change a lot of .travis.yml to php: 7.4 once it's released, but on balance it's better to get a heads up on any potential PHP 7.4 issues than being caught out by them once the stable release is out Travis now supports PHP 7.4 stable, so we can target this

Tasks

• [x] Apply PHP 7.4 build to the 4.5 / 4 / master branches of every module contained in silverstripe/installer (ensure all builds are green before merging these PRs)
• [x] Smoke test a 4.x install of cwp/cwp-recipe-kitchen-sink
• [x] Raise PRs to apply PHP 7.4 build to the 3.7 / 3 branches of every module contained in silverstripe/installer
• [x] Raise issue to resolve failing builds on 7.4 PRs to 3.7 / 3

PRs

4.x

• [x] [silverstripe/assets](https://github.com/silverstripe/silverstripe-assets/pull/376) merged up
• [x] [silverstripe/config](https://github.com/silverstripe/silverstripe-config/pull/40) merged up
• [x] [silverstripe/framework](https://github.com/silverstripe/silverstripe-framework/pull/9412) merged up
• [x] [silverstripe/admin](https://github.com/silverstripe/silverstripe-admin/pull/1014) merged up
• [x] [silverstripe/asset-admin](https://github.com/silverstripe/silverstripe-asset-admin/pull/1064) merged up
• [x] [silverstripe/campaign-admin](https://github.com/silverstripe/silverstripe-campaign-admin/pull/157) merged up
• [x] [silverstripe/versioned-admin](https://github.com/silverstripe/silverstripe-versioned-admin/pull/150) merged up
• [x] [silverstripe/cms](https://github.com/silverstripe/silverstripe-cms/pull/2528) merged up
• [x] [silverstripe/errorpage](https://github.com/silverstripe/silverstripe-errorpage/pull/48) merged up
• [x] [silverstripe/graphql](https://github.com/silverstripe/silverstripe-graphql/pull/251) merged up
• [x] [silverstripe/reports](https://github.com/silverstripe/silverstripe-reports/pull/124) merged up
• [x] [silverstripe/siteconfig](https://github.com/silverstripe/silverstripe-siteconfig/pull/109) merged up
• [x] [silverstripe/versioned](https://github.com/silverstripe/silverstripe-versioned/pull/266) merged up
• [x] [silverstripe/recipe-core](https://github.com/silverstripe/recipe-core/pull/52) merged up
• [x] [silverstripe/recipe-cms](https://github.com/silverstripe/recipe-cms/pull/31) merged up
• [x] [silverstripe/installer](https://github.com/silverstripe/silverstripe-installer/pull/275) merged up

3.x

• [x] [silverstripe/framework](https://github.com/silverstripe/silverstripe-framework/pull/9420)
• [x] [silverstripe/cms](https://github.com/silverstripe/silverstripe-cms/pull/2529)
• [x] [silverstripe/reports](https://github.com/silverstripe/silverstripe-reports/pull/125)
• [x] [silverstripe/siteconfig](https://github.com/silverstripe/silverstripe-siteconfig/pull/110)

[chillu]

PHP 7.4 has been released today

[brynwhyman]

The following WIP PR will merge 7.4 builds into our kitchen sink test suite: https://github.com/silverstripe/cwp-recipe-kitchen-sink/pull/49 (although we'll still want to update the core modules separately)

[chillu]

Just to clarify: Are we doing this for module both on the 1.x and 2.x recipe? We've already got 7.4 builds on framework:3.x (see issue) so sounds like that decision has been made at least for the module already.

[brynwhyman]

@chillu I think we should be looking at 7.4 compatibility for v3.x (and the CWP 1.x recipe) as well, especially if the current test failures look to only be as a result of PHP depreciations.

[Cheddam]

PRs for both 4.x and 3.x have been raised, with the 4.x ones already merged. I've raised a follow-up issue to get the 3.x PRs green and merged: #9421

Remaining effort is merging up 4.5 branches to 4 and master.