Meta: Rename PR templates to be more descriptive in the GitHub UI

[CAM-Gerlach]

Followup to #2956 , as there was no way to test any of this before pushing it live to the production repo. This:

• Remove the data block at the top of each template that apparently isn't currently used by the present GitHub implementation of the feature
• Gives the templates more descriptive and human-readable names since it's the filenames that are displayed directly in the UI.
• Is clearer about the purpose of the Other template (Infra/Meta), since GitHub also provides a blank option
• Also trims an unnecessary title heading from most of the templates to slim them down further

Hopefully we can merge this quickly to avoid this issue lingering for anyone who submits a PR.

[hugovk]

Merge when ready 👍

[CAM-Gerlach]

Looking better now, thanks:

Only one little tweak—the "Other" entry needs the "Other" to sort properly and not look out of place with the, erm, others. I opened #3000 to do that.

[CAM-Gerlach]

Err, #3001 now