[local_auth] Update app facing package to use default method channel implementation in new platform interface package.

[BeMacized]

This PR updates the app facing package to switch over to the default method channel implementation provided by the new platform interface package.

This is a precursor PR to #4699, #4700 and #4701, and is required to fix the integation tests running properly with those PRs.

Relevant issues:

• Federate the Android and iOS implementations of 1P plugins flutter#68498
• Eliminate use of shared method channels in flutter/plugins flutter#94224

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter. (Unlike the flutter/flutter repo, the flutter/plugins repo does use dart format.)
• I signed the CLA.
• The title of the PR starts with the name of the plugin surrounded by square brackets, e.g. [shared_preferences]
• I listed 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 this PR is exempt from version changes.
  • Did not update version number, listed change under NEXT. This will be updated by [local_auth] Refactor package to make use of new platform interface and native implementations #4701. Should this already be done here instead?
• I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

[stuartmorgan-g]

We should remember to make the version update a breaking change when re-publishing this plugin, since this is where we're introducing the new enum values. (Which doesn't necessarily have to be a breaking change, but I think that's the best way to do it.)

[BeMacized]

Sounds like a plan, will update this in #4701.

[stuartmorgan-g]

LGTM once this is updated to published version of #5196

[BeMacized]

@stuartmorgan I'd mark this to be merged, but legacy-version-analyze times out for a reason unknown to me.

[fluttergithubbot]

This pull request is not suitable for automatic merging in its current state.

• The status or check suite legacy-version-analyze has failed. Please fix the issues identified (or deflake) before re-applying this label.

[stuartmorgan-g]

@stuartmorgan I'd mark this to be merged, but legacy-version-analyze times out for a reason unknown to me.

For some reason when flutter pub get can't resolve, it manifests on bots as a timeout instead of just failing and logging. So mystery hangs in the build step are usually due to a resolution failure.

In this case, local_auth_platform_interface requires 2.8.0, so this won't resolve on 2.5.0. You just need to bump the app-facing package to 2.8.0 in this PR so that the 2.5 legacy test will be skipped.