Remove CODEOWNERS

[stuartmorgan-g]

CODEOWNERS was originally added to deal with the problem of routing reviews to the right people, given the highly disparate ownership of packages in this repo. However, it has always had some significant drawbacks due the rigid implementation GitHub has for it, and some are getting worse:

• Low-quality PRs that should be handled in initial triage were going directly to individual reviewers, who don't necessarily know what kinds of pre-review filters to apply, causing some wasted reviewer effort
  • The increase of agent-generated PRs is making this worse, and that trend will almost certainly continue to increase
• Some PRs (e.g., rolling all the minimum SDK versions when a new stable is released) spam a lot of people who don't actually need to do the review, again wasting some people's time.
  • The way the new batch release process merges changes across branches will make this much more common.
• Plugin PRs are routed to all reviewers at once. In some cases platform reviewers would review implementations before the cross-platform owner reviewed, and either asked for fundamental API changes, or decided it wasn't something we should do at all, which wastes reviewer time and is a terrible experience for the contributor. Ideally the cross-platform owners should sanity-check the proposed change before looping in platform implementation reviewers.
• It's impossible to remove people who don't need to review for whatever reason. This means the state of the Reviewers section in a PR's sidebar often doesn't (and can't) reflect reality, making triage of current PR states much harder.

The triage-<subeam> label system that we have added more recently solves a lot of the problems that CODEOWNERS was meant to solve in the first place, without any of the downsides. It will also bring flutter/packages PR handling more in line with flutter/flutter PR handling, which makes our processes more coherent.

Rather than eliminating the file entirely, I turned it into a markdown file that we can refer to. I no longer have repo tool enforcement however, to allow for changes in structure over time (e.g., for some low-traffic subteam-owned packages, they may prefer not have specific owners, and just triage to individuals in an ad-hoc way in their subteam triage). As a starting point it has the same people that the CODEOWNERs did, and we can iterate from there (or remove it later if it turns out that keeping it up to date isn't a good use of time).

[stuartmorgan-g]

I did let Antigravity autocomplete do a bunch of the tedious format transformation via autocomplete once it figured out what I was doing. I was watching the suggested transformations, and did some spot-checking, but consider casting an eye over the parts of the file relevant to you to make sure it didn't mess up any owner mapping that I missed.

[gemini-code-assist]

Code Review

This pull request replaces the CODEOWNERS file with a new SUGGESTED_REVIEWERS.md file, updating the repository's package ownership and review documentation. Corresponding changes were made to the RepoPackageInfoCheckCommand to remove logic related to CODEOWNERS validation, and various test files were updated to reflect the removal of CODEOWNERS and the inclusion of the new file. A minor improvement was suggested to correct the formatting of a package name in the new documentation file.