perf: replace goheader linter with custom check (#37599)

Replace the [slow `goheader` linter](https://github.com/denis-tingaikin/go-header/issues/70) with a
custom check.

Local go lint time is down from 247s to 32s. 6 new files that were
previously undetected because of `//go:build ignore` are fixed. The exit
code of the make target preserves the golangci-lint exit code, if
present.

Also refactors and consolidates the linting targets.

Signed-off-by: silverwind <me@silverwind.io>
Signed-off-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: Claude (Opus 4.7) <noreply@anthropic.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: Giteabot <teabot@gitea.io>

.golangci.yml

@@ -13,7 +13,6 @@ linters:
     - forbidigo
     - gocheckcompilerdirectives
     - gocritic
-    - goheader
     - govet
     - ineffassign
     - mirror
@@ -118,11 +117,6 @@ linters:
       enable:
         - nilness
         - unusedwrite
-    goheader:
-      values:
-        regexp:
-          HEADER: '((Copyright [^\n]+|All rights reserved\.)\n)*Copyright \d{4} (The (Gogs|Gitea) Authors|Gitea Authors|Gitea)\.( All rights reserved\.)?(\n(Copyright [^\n]+|All rights reserved\.))*\nSPDX-License-Identifier: [\w.-]+'
-      template: '{{ HEADER }}'
   exclusions:
     generated: lax
     presets: