Fixing token validity verification that was causing authentication error

[ana-vinogradova-camunda]

Fix for this issue

• Before the fix: token was still considered valid for 30 seconds after its expiration. That was resulting in UNAUTHENTICATED error.
• The fix: method to check for token validity returns false if token already expired by now.

Related issues

closes #17799

[backport-action]

Successfully created backport PR for stable/8.5:

• 18202