Add client/pkg/.gomodguard.yaml and prevent pkg, api and client/v3 from depending on server/v3

[ahrtr]

followup to #20748

Please read https://github.com/etcd-io/etcd/blob/main/CONTRIBUTING.md#contribution-flow.

[k8s-ci-robot]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 69.24%. Comparing base (178fcd0) to head (9a4481e).

Additional details and impacted files

see 42 files with indirect coverage changes

@@            Coverage Diff             @@
##             main   #20755      +/-   ##
==========================================
+ Coverage   66.24%   69.24%   +3.00%     
==========================================
  Files         422      422              
  Lines       34819    34819              
==========================================
+ Hits        23065    24112    +1047     
+ Misses      10377     9317    -1060     
- Partials     1377     1390      +13     

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[ahrtr]

/test pull-etcd-e2e-amd64