Update link branches to `main`

[guidezpl]

• Update CS and googlesource.com link branches
• Update GitHub /blob/ and /tree/ links

Tested links manually and fixes a few broken or deprecated links

Added a test that validates that master isn't used, except for specified repos.

Part of https://github.com/flutter/flutter/issues/121564

Pre-launch Checklist

• [x] I read the Contributor Guide and followed the process outlined there for submitting PRs.
• [x] I read the Tree Hygiene wiki page, which explains my responsibilities.
• [x] I read the Flutter Style Guide recently, and have followed its advice.
• [x] I signed the CLA.
• [x] I listed at least one issue that this PR fixes in the description above.
• [x] I updated/added relevant documentation (doc comments with ///).
• [x] I added new tests to check the change I am making, or this PR is test-exempt.
• [x] All existing and new tests are passing.

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