chore: depend on protected branches of rcgen, rustls-webpki, rustls

[mkmks]

Description of changes

This is a followup to #437. Instead of depending on feature branches of rcgen, webpki and rustls, we should depend on the protected branches (rcgen/main, webpki/rel-0.103, rustls/rel-0.23).

PR Checklist

I attest that all checked items are satisfied. Any deviation is clearly justified above.

• Title follows conventional commits (e.g. chore: ...).
• Tests added for every new pub item and test coverage has not decreased.
• Public APIs and non-obvious logic documented; unfinished work marked as TODO(#issue).
• unwrap/expect/panic only in tests or for invariant bugs (documented if present).
• No dependency version changes OR (if changed) only minimal required fixes.
• No architectural protocol changes OR linked spec PR/issue provided.
• No breaking deployment config changes OR devops label + infra notified + infra-team reviewer assigned.
• No breaking gRPC / serialized data changes OR commit marked with ! and affected teams notified.
• No modifications to existing versionized structs OR backward compatibility tests updated.
• No critical business logic / crypto changes OR ≥2 reviewers assigned.
• No new sensitive data fields added OR Zeroize + ZeroizeOnDrop implemented.
• No new public storage data OR data is verifiable (signature / digest).
• No unsafe; if unavoidable: minimal, justified, documented, and test/fuzz covered.
• Strongly typed boundaries: typed inputs validated at the edge; no untyped values or errors cross modules.
• Self-review completed.

[eudelins-zama]

LGTM