Remove support for detached materials

[woodruffw]

Detached materials have been deprecated in favor of Sigstore bundles (.sigstore.json or .sigstore) for a long time now. We should remove support from the sign and verify commands entirely.

#718 is the main blocker for this, since we'll redirect users towards creating a bundle from their old detached materials.

Related:

• Deprecate the --cert alias in favor of --certificate #149
• Ergonomic way to request certificate/signature files for multiple input files in v2 #771

[woodruffw]

Assigning this to the next major release, since this will be a major breaking change.