Remove instant provisioning for exams

[mrgnr]

Done

• Remove instant provisioning for exams
• Update text on scheduling page to inform users that exams can be scheduled 30 mins or more into the future

QA

• Visit https://ubuntu-com-13045.demos.haus/credentials/your-exams
  • Verify that there is no "Take now" button for redeemed exams

Issue / Card

Fixes https://warthogs.atlassian.net/browse/WD-5127

Screenshots

credentials/your-exams:

credentials/schedule:

Help

QA steps - Commit guidelines

[webteam-app]

Demo starting at https://ubuntu-com-13045.demos.haus

[codecov]

Codecov Report

Merging #13045 (6733bbd) into main (6b288d3) will increase coverage by 74.90%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           main   #13045       +/-   ##
=========================================
+ Coverage      0   74.90%   +74.90%     
=========================================
  Files         0      107      +107     
  Lines         0     2873     +2873     
  Branches      0      928      +928     
=========================================
+ Hits          0     2152     +2152     
- Misses        0      699      +699     
- Partials      0       22       +22     

see 107 files with indirect coverage changes