[v11.0/forgejo] fix: use linguist-generated for language stats (#7694)

**Backport:** https://codeberg.org/forgejo/forgejo/pulls/7685

- Adds code comment to explain behavior of the `linguist-generated` gitattribute.
- Adjusts the code to ignore the file if `linguist-generated` is true.
- Resolves forgejo/forgejo#7677
- Adds unit testing.

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## Release notes
<!--URL:https://codeberg.org/forgejo/forgejo-->
- Bug fixes
  - [PR](https://codeberg.org/forgejo/forgejo/pulls/7694): <!--number 7694 --><!--line 0 --><!--description Zml4OiB1c2UgYGxpbmd1aXN0LWdlbmVyYXRlZGAgZm9yIGxhbmd1YWdlIHN0YXRz-->fix: use `linguist-generated` for language stats<!--description-->
<!--end release-notes-assistant-->

Co-authored-by: Gusted <postmaster@gusted.xyz>
Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/7694
Reviewed-by: Gusted <gusted@noreply.codeberg.org>
Co-authored-by: forgejo-backport-action <forgejo-backport-action@noreply.codeberg.org>
Co-committed-by: forgejo-backport-action <forgejo-backport-action@noreply.codeberg.org>

modules/git/repo_language_stats_test.go

@@ -25,6 +25,15 @@ func TestRepository_GetLanguageStats(t *testing.T) {
 		"Python": 134,
 		"Java":   112,
 	}, stats)
+
+	stats, err = gitRepo.GetLanguageStats("95d3505f2db273e40be79f84416051ae85e9ea0d")
+	require.NoError(t, err)
+
+	assert.Equal(t, map[string]int64{
+		"Cobra":  67,
+		"Python": 67,
+		"Java":   112,
+	}, stats)
 }
 
 func TestMergeLanguageStats(t *testing.T) {