ci(all): Migrate to Melos 3

[spydon]

Description

This migrates the repository and pipeline over to Melos 3.
The contributing guide doesn't have to be updated since that already installs the latest Melos version.

Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I titled the PR using Conventional Commits.
• I did not modify the CHANGELOG.md nor the pubspec.yaml files. (Well, I did add the root pubspec file)
• All existing and new tests are passing.
• The analyzer (flutter analyze) does not report any problems on my PR.

Breaking Change

Does your PR require plugin users to manually update their apps to accommodate your change?

• No, this is not a breaking change.

[spydon]

@vbuberen I saw that you pinned the Melos version to 2.9.0 previously, I'm guessing it was just because you didn't have time to do the migration directly, or was there any other reason?

[vbuberen]

The reason was to not bump min Dart version to 2.18 as it is needed for Melos 3.0.0: https://github.com/invertase/melos/blob/main/CHANGELOG.md#melos---v300
Explained it in this PR: #1615

[vbuberen]

Replied in #1615 that I missed the fact about separate pubspec.yaml for workspace. Looks good to me then.
Thanks for taking care of it. I would bump min Dart for that workspace pubspec, considering that it doesn't affect packages/users.

[vbuberen]

I tested this PR a bit earlier today and everything works fine.
We still have the issue with publishing: invertase/melos#467 but it is not Melos, but pub as it seems to me.

[spydon]

I tested this PR a bit earlier today and everything works fine. We still have the issue with publishing: invertase/melos#467 but it is not Melos, but pub as it seems to me.

Was it the examples' pubspec_overrides that were breaking it? If so I think I fixed it here:
#1628 :)

[vbuberen]

Was it the examples' pubspec_overrides that were breaking it? If so I think I fixed it here:
#1628 :)

No, it is not. Replied in that other PR.