Demonstrations of how to add a plugin dependency in pubspec.yaml
inevitably become outdated, causing more confusion than they solve.
pub.dev has an installation tab which shows the same example but using a
dynamic version, so these instructions are almost never necessary; where
specifically referencing installation is useful, link to that tab
instead of an inline, hard-coded example.

Many of these examples were in implementation packages that were not
endorsed, and had never been updated to indicate that they are now
endorsed, so this PR also corrects those READMEs to indicate that they
are in fact endorsed.

Fixes flutter/flutter#87056

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. (Note that 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.
• 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.