build: update Go to 1.27.1 and modernize the codebase (#2798)

Toolchain and CI:
- Bump go directive from 1.26.8 to 1.27.1 (go.mod)
- Use golang:1.27-alpine builder images in all Dockerfiles
- Bump golangci-lint to v2.13.2 (first release with go1.27 support)
- Add CI gate: `go fix -diff` fails when outdated Go patterns are
  detected (locally: `make check-modernize`)

Note: darwin binaries now require macOS 13 or later.

Lint fixes required by golangci-lint v2.13.2:
- goconst: ignore tests (all 436 findings were test-only; goconst
  got stricter since v2.12 and this option was added for it)
- openai.go: keep deprecated MaxTokens deliberately with a nolint
  rationale (max_tokens is the only form universally supported by
  OpenAI-compatible backends like One-API, LiteLLM, Ollama shim)
- state.go: drop always-nil flags param from appendChartVersionFlags
  (renamed to chartVersionFlags, unparam)

Modernization (go fix ./..., 62 files):
- interface{} -> any, maps.Copy, strings.SplitSeq, range-over-int,
  builtin min/max, slices.Contains/ContainsFunc/Sort, WaitGroup.Go,
  reflect.Type.Fields(), new(expr)
- exit_error.go: strings.Builder + fmt.Fprintf instead of string
  concatenation and WriteString(fmt.Sprintf(...)) (QF1012)
- chart_dependency.go: strings.CutLast for OCI dependency helpers

Signed-off-by: yxxhero <aiopsclub@163.com>
Co-authored-by: Claude <noreply@anthropic.com>

Makefile

@@ -38,6 +38,12 @@ check:
 	go vet ${PKGS}
 .PHONY: check
 
+# check-modernize fails (exit 1) when any `go fix` modernizer would rewrite code,
+# i.e. the codebase contains outdated Go patterns. Run `go fix ./...` to fix, then `make fmt`.
+check-modernize:
+	go fix -diff ./...
+.PHONY: check-modernize
+
 build-test-tools:
 	go build test/diff-yamls/diff-yamls.go
 	curl --progress-bar --location https://github.com/homeport/dyff/releases/download/v1.5.6/dyff_1.5.6_linux_amd64.tar.gz  | tar -xzf - -C `pwd` dyff