PMM-7 Fix linter config and some errors

[ademidoff]

PMM-7

This PR:
1/ fixes the linter configuration for the depguard rule
2/ fixes some - not all (!) - linter errors

Team, feel free to contribute to more linter error fixes 👋

[codecov]

Codecov Report

Merging #2504 (3f35176) into main (7678adc) will decrease coverage by 0.01%.
The diff coverage is 25.00%.

@@            Coverage Diff             @@
##             main    #2504      +/-   ##
==========================================
- Coverage   42.82%   42.82%   -0.01%     
==========================================
  Files         384      384              
  Lines       48348    48350       +2     
==========================================
  Hits        20707    20707              
- Misses      25689    25693       +4     
+ Partials     1952     1950       -2     

Flag	Coverage Δ
admin	10.41% <ø> (-0.05%)	⬇️
agent	53.15% <0.00%> (ø)
managed	43.89% <33.33%> (+<0.01%)	⬆️
vmproxy	68.57% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
managed/services/agents/grpc/agent_server.go	0.00% <ø> (ø)
managed/services/agents/log_level.go	36.36% <ø> (ø)
managed/services/backup/backup_service.go	58.60% <ø> (ø)
managed/services/backup/compatibility_service.go	58.78% <ø> (ø)
managed/services/dbaas/kubernetes/client/client.go	3.60% <ø> (ø)
managed/services/dbaas/kubernetes/kubernetes.go	10.51% <ø> (ø)
...aged/services/management/backup/backups_service.go	56.54% <ø> (ø)
managed/services/management/grpc/actions_server.go	0.00% <ø> (ø)
managed/services/management/grpc/mongodb_server.go	0.00% <ø> (ø)
managed/services/management/grpc/mysql_server.go	0.00% <ø> (ø)
... and 7 more

... and 1 file with indirect coverage changes

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