[Merged by Bors] - chore(Tactic): move StacksAttribute to CrossRefAttribute

[jcommelin]

This PR moves StacksAttribute.lean to CrossRefAttribute.lean
both in Mathlib/Tactic/ and in MathlibTest/.

In the future, we can refactor the file to abstract over the type of
cross-references. Currently, cross-reference attributes are implemented
for the Stacks Project and Kerodon.
A good candidate for a future PR is to add an attribute for Wikidata
identifiers.

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[github-actions]

PR summary 53f8a93a77

Import changes for modified files

No significant changes to the import graph

Import changes for all files

Files	Import difference
Mathlib.Tactic.StacksAttribute	-54
Mathlib.Tactic.CrossRefAttribute	54

---

Declarations diff

No declarations were harmed in the making of this PR! 🐙

You can run this locally as follows

## from your `mathlib4` directory:
git clone https://github.com/leanprover-community/mathlib-ci.git ../mathlib-ci

## summary with just the declaration names:
../mathlib-ci/scripts/pr_summary/declarations_diff.sh <optional_commit>

## more verbose report:
../mathlib-ci/scripts/pr_summary/declarations_diff.sh long <optional_commit>

The doc-module for scripts/pr_summary/declarations_diff.sh in the mathlib-ci repository contains some details about this script.

---

No changes to strong technical debt.

No changes to weak technical debt.

---

note: file MathlibTest/StacksAttribute.lean` was renamed to `MathlibTest/CrossRefAttribute.lean without a module deprecation
Please create a follow-up pull request adding one. Thanks!

note: file Mathlib/Tactic/StacksAttribute.lean` was renamed to `Mathlib/Tactic/CrossRefAttribute.lean without a module deprecation
Please create a follow-up pull request adding one. Thanks!

[adomani]

bors d+

[mathlib-bors]

✌️ jcommelin can now approve this pull request. To approve and merge a pull request, simply reply with bors r+. More detailed instructions are available here.

[grunweg]

Thanks for getting this started!
In my PR, I went to the name "DatabaseAttributes". I don't think that's better than this one, though.

[mathlib-auto-merge]

As this PR is labelled auto-merge-after-CI, we are now sending it to bors:

bors merge

[mathlib-bors]

Pull request successfully merged into master.

Build succeeded:

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