[workspace] Deprecate the petsc external

[jwnimmer-tri]

We anticipate no longer using this soon, so it's time to start the clock for its removal.

+@xuchenhan-tri for both reviews, please.

Towards #17231 and #19882.

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This change is 

[xuchenhan-tri]

To check my understanding: this has no observable effect when linking against PETSc internally (other than switching the spelling to petsc_nowarn) and we can remove the dependency entirely when we don't even need PETSc for internal use. Is that right?

Reviewed 4 of 4 files at r1, all commit messages.
Reviewable status: LGTM missing from assignee xuchenhan-tri(platform)

[jwnimmer-tri]

This has no observable effect when linking against PETSc internally (other than switching the spelling to petsc_nowarn) ...

Yes. There is no functional effect. The build system will run a few extra commands due to the "nowarn" shim, but nothing important.

... we can remove the dependency entirely when we don't even need PETSc for internal use.

We can delete tools/workspace/petsc after both conditions are satisfied:
(1) FEM no longer uses it, and
(2) the deprecation date has passed.

As of (1) we can drop the deps line in the fem BUILD file, but we need to keep the tools/workspace around until the deprecation date has passed. (Workspace rules are Stable API, unless marked internal.)

[xuchenhan-tri]

Understood. Thanks for the explanation.

x2

Reviewable status: complete! all discussions resolved, LGTM from assignee xuchenhan-tri(platform)