fix(android): build on Firebase BoM 35 and require minSdk 24

[SelaseKay]

Description

Stacked on #18767. The base branch is feat/bump-android-sdk-35.0.0, not main, because these changes only compile against the Firebase Android BoM 35 pin. Merge this into the bump branch first; that turns #18767 green, and then it merges into main.

• Firestore and ML Model Downloader dependencies. BoM 35 no longer manages firebase-ml-modeldownloader, so the plugin now pins it explicitly at 26.1.1, the version BoM 34.19.0 resolved. It will no longer follow BoM bumps. Firestore 27.0.0 publishes mockito-core, junit, truth and jackson-databind as runtime dependencies; Mockito pulls byte-buddy 1.17.7, which Jetifier cannot transform, so the Firestore plugin excludes them.
• minSdk 24. BoM 35 raises the Firebase libraries to API 24. Plugin configs and example apps that still pinned 23 now require 24.
• Example apps. Flutter 3.27 through 3.32 still default flutter.minSdkVersion to 21, so example apps that follow it now floor at 24 to pass manifest merger.

Related Issues

• Part of 🚀 [firebase_core] Bump Firebase Android SDK (34.19.0 → 35.0.0) #18765

Checklist

Before you create this PR confirm that it meets all requirements listed below by checking the relevant checkboxes ([x]).
This will ensure a smooth and quick review process. Updating the pubspec.yaml and changelogs is not required.

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• My PR includes unit or integration tests for all changed/updated/fixed behaviors (See Contributor Guide).
• All existing and new tests are passing.
• I updated/added relevant documentation (doc comments with ///).
• The analyzer (melos run analyze) does not report any problems on my PR.
• I read and followed the Flutter Style Guide.
• I signed the CLA.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

Does your PR require plugin users to manually update their apps to accommodate your change?

• Yes, this is a breaking change.
• No, this is not a breaking change.

Apps must use Android minSdk 24 or higher. The breaking-change marker for the release lives on the title of #18767, because this pull request is squashed into that branch.

Test plan

• CI on this pull request runs the Android jobs for the files it touches.
• After it merges, the full CI run on feat!: bump Firebase Android SDK to 35.0.0 #18767 should be green on Android.

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