[META] Use correct language for .tmpl

- Upstream chosen `Handlebars` as language for the templates, presumenly
because Github doesn't have an syntax highlighter for Go.
- Forgejo does have syntax highlighting support for Go templates, so use that.

(cherry picked from commit a79718522dc432e32cf322498ddf22ec570fa8fd)
(cherry picked from commit 380d847e5cdb179292c7bd1a5e3e6839fb8b31dd)
(cherry picked from commit 4126a9672d93fc33b72fb816a404056aac64dfc8)
(cherry picked from commit afde0ed82271df540348953a811a8e9db1a83343)
(cherry picked from commit 6ddaca00615340958e1aecca2c4807e39a2b1eff)
(cherry picked from commit 454b2f6b35c018cc631dd8847ae9d3243d9adc37)
(cherry picked from commit 58c10e1f93da8d8f193936eff00ebd86ecd8b78c)

.gitattributes

@@ -1,5 +1,5 @@
 * text=auto eol=lf
-*.tmpl linguist-language=Handlebars
+*.tmpl linguist-language=go-html-template
 /assets/*.json linguist-generated
 /public/assets/img/svg/*.svg linguist-generated
 /templates/swagger/v1_json.tmpl linguist-generated