[3006.x] Migrate to TZ-aware datetime objects in x509_v2

[lkubb]

What does this PR do?

• replaces datetime.utcnow with datetime.now(tz=timezone.utc)
• generally uses TZ-aware datetimes internally
• thus also fixes a bug in CRL creation (see linked issue)

What issues does this PR fix or reference?

Fixes: https://github.com/saltstack/salt/issues/65837 Ref: https://github.com/saltstack/salt/issues/65604

Previous Behavior

CRLs were created with wrong timestamps, Python 3.12 would log DeprecationWarnings

New Behavior

Correct CRL timestamps, no warnings

Merge requirements satisfied?

• [ ] Docs
• [x] Changelog - https://docs.saltproject.io/en/master/topics/development/changelog.html
• [x] Tests written/updated ~~- not sure how to test the issue reliably tbh. A regression is unlikely since we're dealing in TZ-aware datetimes only now.~~

Commits signed with GPG?

Yes

[s0undt3ch]

@dwoz can you review this one again please?

[lkubb]

Sorry for dismissing the review, had to resolve conflicts after the idna changes.

[lkubb]

@dwoz Anything holding this up? It should be quite straight-forward.