feat: enable modernize linter in golangci-lint and fix all violations

[mmorel-35]

Please add a summary of your change

Enables the modernize linter in the golangci-lint configuration and fixes all reported violations across the codebase. The modernize linter is part of golangci-lint since v2.1.0 and the project already uses v2.12.0, so no tooling version bump is needed.

Changes to .golangci.yaml

• Added modernize to the linters.enable list
• Disabled the newexpr check via linters.settings.modernize.disable (placed in alphabetical order within linters.settings) — the golangci-lint auto-fix for newexpr generates syntactically invalid Go code (new(literal_value) — new only accepts types, not values), making it unsafe to enable

Fixes applied across 49 Go files

Check	Description	Files touched
mapsloop	Replace for k, v := range src { dst[k] = v } with maps.Copy	10 files
slicescontains	Replace manual contains loops with slices.Contains	6 files
stringsseq	Replace for _, s := range strings.Split(...) with strings.SplitSeq	7 files
stringscut	Replace strings.Index patterns with strings.Cut	3 files
rangeint	Replace for i := 0; i < n; i++ {} with for range n {}	8 files
fmtappendf	Replace []byte(fmt.Sprintf...) with fmt.Appendf(nil, ...)	2 files
plusbuild	Remove legacy // +build lines (replaced by //go:build)	3 files
minmax	Replace if/else patterns with max()/min() builtins	1 file
omitzero	Replace omitempty (no-op for struct fields) with omitzero	2 files
slicesbackward	Replace backward loops with slices.Backward	2 files
waitgroupgo	Replace wg.Add(1); go func() { defer wg.Done() ... }() with wg.Go(...)	1 file
atomictypes	Replace var x int32 + atomic.*Int32 calls with atomic.Int32	1 file

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• 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