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

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.