Gatsby: Upgrade `serve` dependency to version 7

[jlengstorf]

Description

There's a security vulnerability in serve that GitHub just started flagging. The fix is to upgrade serve to serve@^7.0.0. Currently we're still on version 6.

https://github.com/gatsbyjs/gatsby/blob/d8aaa2f346ea30fac760b0fb4359bb251779525a/packages/gatsby/package.json#L117

Steps to Solve

• [ ] Run yarn install serve@latest in the gatsby package
• [ ] Ensure gatsby serve continues to work as expected
• [ ] Spread the word that everyone should upgrade to remove the vulnerability

We should apply this upgrade to both v1 and v2, assuming there are no breaking changes for v1.

[m-allanson]

The latest version is 9.2.0, so it's probably worth updating all the way up to that.

[m-allanson]

This was handled in the above mentioned PRs, thanks folks!