deprecate: Discard the existence of [project] and change WARNING to ERROR

[heisen-li]

What does this PR try to resolve?

a part of #13629

Maybe start this now? It's always good to get rid of something：)

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[epage]

Couple of notes

• fix(toml): Error on [project] in Edition 2024 #13747 is also open for this
• The next steps on that issue are proposals, we still need to check with the team on them (I made a guess with fix(toml): Error on [project] in Edition 2024 #13747 that our next steps would be approved)
• As a heads up, "deprecating" means to discourage the use of something and that it no longer gets new features, rather than remove support for it.

[bors]

☔ The latest upstream changes (presumably #13747) made this pull request unmergeable. Please resolve the merge conflicts.

[weihanglo]

Close as #13747 was just merged.