Consolidate release-note labels onto GitHub defaults

[frazane]

Cleans up the label set introduced with the release-notes automation. The custom labels fix / feature / docs duplicated GitHub's standard bug / enhancement / documentation, so this standardizes on the defaults and points the automation at them.

What changed

• release.yml: Bug fixes now keys off bug, New features off enhancement, Documentation off documentation (dropping the redundant fix / feature / docs synonyms).
• labeler.yml: the docs path rule now applies the documentation label.
• Pull request template: the release-note label list matches the categories above.

Notes

• The duplicate labels fix / feature / docs are being deleted from the repo as part of this cleanup.
• Existing PRs are being re-labeled to match (documentation instead of docs), and the breaking label is being applied to the Python 3.11 and tensorflow-backend removals so they surface correctly in the next release notes.