fix: support releasing from non-main branches with version override

[luxscious]

Fix release workflow to support non-main release branches

The prepare-release action computes the next version from the most recently created git tag rather than the highest
semver tag. After the v2.3.3 backport (created after v2.6.3), this caused the workflow to compute 2.4.0 as the next
version instead of 2.7.0.

Changes:

• Adds an optional version-override input to manually specify the release version, bypassing the broken auto-increment
  when needed
• Fixes git push origin HEAD:main → git push origin HEAD so the workflow can release from branches other than main
• Changes the GitHub release target from hardcoded main to ${{ github.ref_name }} for the same reason

[Copilot]

Pull request overview

Updates the GitHub Actions release workflow so releases can be cut from non-main branches and (optionally) override the computed next version when the auto-increment logic chooses the wrong base tag.

Changes:

• Add a version-override workflow_dispatch input and a “Resolve version” step that selects either the override or the computed next tag.
• Replace hardcoded main branch assumptions by pushing without :main and creating releases targeting ${{ github.ref_name }}.
• Wire version usage (sed updates, commit message, release tag) through the resolved version output.

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