upgrade to v3: mention MR approvals syntax change

[weakcamel]

Related to #388 and #263

[weakcamel]

After a recent Gitlab instance upgrade I finally got around to upgrading Gitlabform too.

While porting the configs I've noticed that the current version doesn't like the approvals_before_merge outside of merge_requests_approval_rules (see below) so I'm proposing to mention this in the upgrade guide.

Processing section 'merge_requests_approvals'
Error: Setting the 'approvals_before_merge' in the 'merge_requests_approvals' sections is not allowed as it is not clear which rule does it apply to. Please set it inside the specific approval rules in the 'merge_requests_approval_rules' section.

[codecov]

Codecov Report

Merging #525 (13ae508) into main (9bd5880) will decrease coverage by 3.10%.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #525      +/-   ##
==========================================
- Coverage   82.03%   78.94%   -3.10%     
==========================================
  Files          70       70              
  Lines        2650     2650              
==========================================
- Hits         2174     2092      -82     
- Misses        476      558      +82     

see 12 files with indirect coverage changes

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[gdubicki]

Sure, why not. Thanks for the contribution, @weakcamel!