[9.3] .golangci.yml: enable the modernize linter

[orestisfl]

Manual backport of #51840 using the shell script:

#!/usr/bin/env bash
# Reproduces the Go changes of PR1 deterministically.
# Precondition: only .golangci.yml is modified (by hand); the tree is otherwise
# a clean checkout. Run from the repository root.
set -euo pipefail

OWNER="@elastic/elastic-agent-data-plane"
export CGO_ENABLED=1
MAX_PASSES=10

modernize() {
	golangci-lint run --enable-only modernize \
		--max-issues-per-linter 0 --max-same-issues 0 --timeout=90m --fix ./...
}

# Fixpoint: fmt, then modernize, then apply //go:fix inline directives. Each
# step can unlock the next, so iterate until a full pass changes nothing.
for pass in $(seq "$MAX_PASSES"); do
	echo ">>> pass $pass: fmt"
	before="$(git diff | git hash-object --stdin)"
	golangci-lint fmt ./...
	echo ">>> pass $pass: modernize --fix"
	if modernize; then :; fi
	echo ">>> pass $pass: go fix -inline"
	# go fix may exit non-zero when an inline leaves an import unused; the next
	# pass's `golangci-lint fmt` drops those imports, so ignore its status.
	if go fix -inline ./...; then :; fi
	echo ">>> pass $pass done: $(git diff --name-only | wc -l) files changed"
	[ "$before" = "$(git diff | git hash-object --stdin)" ] && { echo ">>> fixpoint at pass $pass"; break; }
done
golangci-lint fmt ./...

# Keep only the .go files owned by $OWNER; revert everyone else's files.
changed="$(mktemp)"
keep="$(mktemp)"
trap 'rm -f "$changed" "$keep"' EXIT
git diff --name-only -- '*.go' | sort -u >"$changed"
git diff --name-only -- '*.go' | xargs -r codeowners -o "$OWNER" \
	| awk '{print $1}' | sort -u >"$keep"
comm -23 "$changed" "$keep" | tr '\n' '\0' | xargs -0 -r git checkout --

echo "PR1 data-plane .go files: $(git diff --name-only -- '*.go' | wc -l)"

[github-actions]

🤖 GitHub comments

Just comment with:

• run docs-build : Re-trigger the docs validation. (use unformatted text in the comment!)
• /test : Run the Buildkite pipeline.

[AndersonQ]

@orestisfl, this PR does not change .golangci.yml as the original did: https://github.com/elastic/beats/pull/51840/changes#diff-6179837f7df53a6f05c522b6b7bb566d484d5465d9894fb04910dd08bb40dcc9

Was it intentional?

[mergify]

This pull request is now in conflicts. Could you fix it? 🙏
To fixup this pull request, you can check out it locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

git fetch upstream
git checkout -b modernize-9.3 upstream/modernize-9.3
git merge upstream/9.3
git push upstream modernize-9.3