Clean up cruft in pubspec.yaml files

[stuartmorgan]

• Standardize on mockito 5.0 now that the conflicts with SDK packages is
  resolved on stable.
• Removes overrides that were only needed for Flutter 2.0
• Update a -nullsafe inter-package dependency
• Update some older federated examples that used dependency_overrides to
  depend on its encloding package by path to the now-standard option of
  listing that path-based dependency directly.

Does not update versions since these changes are all to dev_dependencies and/or examples, so don't need to be published.

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 Flutter Style Guide and the C++, Objective-C, Java style guides. (Note that unlike the flutter/flutter repo, the flutter/plugins repo does use dart format. See plugin_tool 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.
• I updated CHANGELOG.md to add a description of the change.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test exempt.
• All existing and new tests are passing.

[mvanbeusekom]

LGTM