fix: strip fluxcd managed labels

[jkroepke]

running helm diff against fluxcd managed helm release also shows a diff in terms of labels.

---

The current workaround is using

      --suppress-output-line-regex="helm.toolkit.fluxcd.io/name:" \
      --suppress-output-line-regex="helm.toolkit.fluxcd.io/namespace:" \

However, each single resource is present in the output with a suppressed notice.

An alternative approach would be:

      --strip-output-line-regex="helm.toolkit.fluxcd.io/name:" \
      --strip-output-line-regex="helm.toolkit.fluxcd.io/namespace:" \

The strip-output-lines is executed on source and target, before diff. Without any notice on the diff output.

[yxxhero]

@jkroepke lint issue.

[yxxhero]

Error: manifest/util.go:33:1: File is not properly formatted (gci)
pruneNestedMap(metadata, "labels",
^
1 issues:

• gci: 1

Error: issues found
Ran golangci-lint in 22762ms

[yxxhero]

Error: manifest/util.go:34:1: File is not properly formatted (gci)
pruneNestedMap(metadata, "labels",
^
1 issues:

• gci: 1

Error: issues found
Ran golangci-lint in 3195ms

[jkroepke]

@yxxhero this PR does not close #1064