Refactor i18n to locale (#20153)

* Refactor `i18n` to `locale`

- Currently we're using the `i18n` variable naming for the `locale`
struct. This contains locale's specific information and cannot be used
for general i18n purpose, therefore refactoring it to `locale` makes
more sense.
- Ref: https://github.com/go-gitea/gitea/pull/20096#discussion_r906699200

* Update routers/install/install.go

templates/repo/migrate/migrate.tmpl

@@ -11,7 +11,7 @@
 								{{.Title}}
 							</div>
 							<div class="description tc">
-								{{(Printf "repo.migrate.%s.description" .Name) | $.i18n.Tr }}
+								{{(Printf "repo.migrate.%s.description" .Name) | $.locale.Tr }}
 							</div>
 						</div>
 					</a>