PEP 815: Deprecate RECORD.jws and RECORD.p7s

[konstin]

Implement PEP 815: Deprecate RECORD.jws and RECORD.p7s

The changes retain the information about signature files to the extend that a post-PEP 815 installer needs to be aware of them (not mentioned in RECORD), and informs build backend authors that these files must not be created anymore. The remaining information on signature files, now irrelevant, is removed.

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📚 Documentation preview 📚: https://python-packaging-user-guide--1996.org.readthedocs.build/en/1996/

[konstin]

The CI failure looks unrelated:

[8](https://github.com/pypa/packaging.python.org/actions/runs/21475202706/job/61857379659?pr=1996#step:5:429)
(specifications/core-metadata: line  260) broken    https://docutils.sourceforge.io/docs/ref/rst/restructuredtext.html - 403 Client Error: Forbidden for url: https://docutils.sourceforge.io/docs/ref/rst/restructuredtext.html
(      contribute: line  102) broken    https://docutils.sourceforge.io/docs/user/rst/quickref.html - 403 Client Error: Forbidden for url: https://docutils.sourceforge.io/docs/user/rst/quickref.html
(guides/distributing-packages-using-setuptools: line   91) broken    https://docutils.sourceforge.io/rst.html - 403 Client Error: Forbidden for url: https://docutils.sourceforge.io/rst.html
(specifications/core-metadata: line 1076) broken    https://docutils.sourceforge.io/ - 403 Client Error: Forbidden for url: https://docutils.sourceforge.io/