golangci-lint: enable int-conversion and fiximports rule of perfsprint

[mmorel-35]

Thank you for contributing to Velero!

Please add a summary of your change

enable int-conversion and fiximports rule of perfsprint
This is to reduce memory consumption

Does your change fix a particular issue?

No

Please indicate you've done the following:

• Accepted the DCO. Commits without the DCO will delay acceptance.
• Created a changelog file (make new-changelog) or comment /kind changelog-not-required on this PR.
• Updated the corresponding documentation in site/content/docs/main.

[mmorel-35]

/kind changelog-not-required

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 58.96%. Comparing base (a46fef8) to head (6a96acd).
Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/backup/actions/csi/pvc_action.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #8385   +/-   ##
=======================================
  Coverage   58.96%   58.96%           
=======================================
  Files         367      367           
  Lines       38895    38893    -2     
=======================================
  Hits        22933    22933           
+ Misses      14500    14498    -2     
  Partials     1462     1462           

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