[video_player] Add missing swift_version to podspec

[stuartmorgan-g]

Adds a swift_version to the podspec of video_player_avfoundation, to fix a build error when building with CocoaPods in a project that doesn't itself have a Swift version set.

Updates the repo tooling podspec-check command to check for this, so that we don't forget it in future migrations.

Also fixes some missing ignores in podspec-check that caused false positives when running the check locally in a tree that has built plugins.

Fixesflutter/flutter#186232

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Footnotes

1. Regular contributors who have demonstrated familiarity with the repository guidelines only need to comment if the PR is not auto-exempted by repo tooling. ↩↩²

[gemini-code-assist]

Code Review

This pull request updates the video_player_avfoundation package to include a Swift version in its podspec and enhances the podspec-check tool to enforce this requirement for all plugins containing Swift code. The tool's logic was also updated to ignore build directories during file discovery. Review feedback suggests refining the path-checking logic to prevent false positives when the repository resides in a directory named 'build', simplifying path segment comparisons, and making the Swift version regex more specific to avoid matching comments.