[url_launcher] Temporarily restore Android 21 support

[stuartmorgan-g]

The change to drop API 21-23 support was intended to set the minimum Flutter version to 3.35, but didn't actually do so. This reverts the changes to drop 21-23, so that anyone who got the broken version can update to a working version, and then I'll follow up with a new PR that re-lands the change but with the missed SDK constraint change.

Pre-Review Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter.
• I signed the CLA.
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I linked to at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or I have commented below to indicate which version change exemption this PR falls under¹.
• I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style, or I have commented below to indicate which CHANGELOG exemption this PR falls under¹.
• I updated/added any relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or I have commented below to indicate which test exemption this PR falls under¹.
• All existing and new tests are passing.

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 ↩³

[flutter-dashboard]

It looks like this pull request may not have tests. Please make sure to add tests or get an explicit test exemption before merging.

If you are not sure if you need tests, consider this rule of thumb: the purpose of a test is to make sure someone doesn't accidentally revert the fix. Ask yourself, is there anything in your PR that you feel it is important we not accidentally revert back to how it was before your fix?

Reviewers: Read the Tree Hygiene page and make sure this patch meets those guidelines before LGTMing. If you believe this PR qualifies for a test exemption, contact "@test-exemption-reviewer" in the #hackers channel in Discord (don't just cc them here, they won't see it!). The test exemption team is a small volunteer group, so all reviewers should feel empowered to ask for tests, without delegating that responsibility entirely to the test exemption group.

[stuartmorgan-g]

test-exempt: Revert, other than the version and changelog.

[gemini-code-assist]

Code Review

This pull request reverts a previous change that dropped support for Android API levels 21-23, by setting minSdkVersion back to 21. The changes also re-introduce code to handle older API levels in WebViewActivity.java, specifically by adding back the deprecated shouldOverrideUrlLoading method for backward compatibility. My review focuses on ensuring the revert is complete and correct. I've found one area in WebViewActivity.java where the implementation for older APIs seems incomplete, which could lead to incorrect behavior on devices running Android API < 24. My suggestion addresses this to ensure consistent behavior across all supported API levels.

[bparrishMines]

LGTM