#3 ci: move to node24 actions and ubuntu-26.04 runners

Summary

Clears the GitHub Actions deprecation warnings:

• Node.js 20: rubygems/release-gem v1.2.0 → v1.4.1 (its bundled configure-rubygems-credentials moves from Node 20 to Node 24).
• ubuntu-latest → Ubuntu 26 migration: runs-on: ubuntu-latest → ubuntu-26.04.

Workflow files only.

Test plan

• CI is green on this PR
• Run annotations no longer show the Node 20 or ubuntu-latest notices

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Summary by cubic

Clears GitHub Actions deprecation warnings by moving CI to non-deprecated runners and Node 24.

• Updates runs-on: ubuntu-latest to ubuntu-26.04 in ci.yml and push.yml.
• Pins Ruby to minor-only 4.0 because ubuntu-26.04 doesn't provide patch builds.
• Bumps ruby/setup-ruby from v1.302.0 to v1.327.0 for Ubuntu 26.04 support.
• Bumps rubygems/release-gem from v1.2.0 to v1.4.1, which replaces the bundled Node 20 runtime with Node 24.