docs: add notice linking to analysis_server_plugin

[Hassnaa9]

Title:
Fixes #61817
docs: add notice linking to analysis_server_plugin

Description:
This pull request updates the pkg/analyzer_plugin/README.md file to include a note directing users to the analysis_server_plugin package.
The change clarifies where plugin developers should look for the maintained implementation and helps avoid confusion when exploring the analyzer plugin system.

Why:
Developers exploring the analyzer plugin system often miss the fact that the maintained plugin implementation has moved to analysis_server_plugin.
This update improves documentation clarity and developer onboarding.

Related issue:
Fixes [dart-lang/sdk#61817](#61817)

---

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[Hassnaa9]

@FMorschel can you review this plz?

[copybara-service]

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https://dart-review.googlesource.com/c/sdk/+/457580

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[FMorschel]

Het @Hassnaa9 ! Thanks for helping out!

The small nit I'd add here would be to separate the **Notice** line (one empty line before and one after). Only because Markdown sometimes can be a bit weird with new lines but other than that. I'd say it's good 😁!

But unfortunately I'm not from the team and I can't review this for you. I can add reviewers on Gerrit though. I'll do that!

[FMorschel]

Note that you'll need to close the issue manually after that CL lands. If you can ask a reviewer to change the description to contain Fixes: <issue-url>, it may close the issue. Otherwise, it will, as stated there, close this PR and not merge it. The merging happens with Copybara somewhere else. And since this will not be merged, it doesn't close the linked issue.

[copybara-service]

https://dart-review.googlesource.com/c/sdk/+/457580 has been updated with the latest commits from this pull request.

[Hassnaa9]

Note that you'll need to close the issue manually after that CL lands. If you can ask a reviewer to change the description to contain Fixes: <issue-url>, it may close the issue. Otherwise, it will, as stated there, close this PR and not merge it. The merging happens with Copybara somewhere else. And since this will not be merged, it doesn't close the linked issue.

ok ,many thanks for you

[copybara-service]

https://dart-review.googlesource.com/c/sdk/+/457580 has been updated with the latest commits from this pull request.

[copybara-service]

https://dart-review.googlesource.com/c/sdk/+/457580 has been updated with the latest commits from this pull request.