Warehouse: Update "how to request a trove classifier change" help

[story645]

Describe the bug
The link in https://pypi.org/help/#new-classifier to trigger a new trove classifier yields an empty template opened on warehouse:

Expected behavior

1. The template should have the following fields since this is what's always requested:
   
   
   
   • name of the requested classifier
   
   
   • a brief justification of why it is important.
   
   
2. the issue should probably be opened on pypi-support since that's where the issue is immediately moved to

To Reproduce
https://github.com/pypa/warehouse/issues/new?title=Request+trove+classifier&template=new-trove-classifier.md

attn: @brainwane xref: #3231

[di]

We moved the repo for these requests, it should link to https://github.com/pypa/pypi-support/issues/new?labels=classifier+request&template=new-trove-classifier.md&title=Trove+Classifier+Request instead.

[di]

Once https://github.com/pypa/warehouse/pull/7582 is merged, we'll actually want to update this to direct users to create a PR at https://github.com/pypa/trove-classifiers instead.

[pradyunsg]

@di Any reason to not start redirecting requests to that repository already? Given that there's concensus already that we'd want to switch to using the library in warehouse.

[di]

Yeah, I'd like to at least get some reviews on #7582 first, to make sure that's headed in the right direction.

[brainwane]

Now that we have merged #7582 I do think redirecting requests to https://github.com/pypa/trove-classifiers is the right move (plus updating https://pypi.org/help/#new-classifier ).