ci: create GitHub releases from version tags

[kim-em]

Summary

This PR adds a GitHub Actions workflow that automatically creates GitHub releases when version tags are pushed.

Currently mathlib4 only has tags (no GitHub releases), which makes it harder to programmatically discover the latest release version. This mirrors the existing functionality in batteries' docs-release.yml, but without building/attaching docs.

See also leanprover/cslib#340

Closes #11292

🤖 Prepared with Claude Code

[github-actions]

PR summary 32fe3f9354

Import changes for modified files

No significant changes to the import graph

Import changes for all files

Files	Import difference

---

Declarations diff

No declarations were harmed in the making of this PR! 🐙

You can run this locally as follows

## summary with just the declaration names:
./scripts/declarations_diff.sh <optional_commit>

## more verbose report:
./scripts/declarations_diff.sh long <optional_commit>

The doc-module for scripts/declarations_diff.sh contains some details about this script.

---

No changes to technical debt.

You can run this locally as

./scripts/technical-debt-metrics.sh pr_summary

• The relative value is the weighted sum of the differences with weight given by the inverse of the current value of the statistic.
• The absolute value is the relative value divided by the total sum of the inverses of the current values (i.e. the weighted average of the differences).

[bryangingechen]

Suggested change

	uses: softprops/action-gh-release@v2
	uses: softprops/action-gh-release@a06a81a03ee405af7f2048a818ed3f03bbf83c7b # v2.5.0

Thanks!
bors d+

[mathlib-bors]

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