Clarify "text-align" CSS helpers, fix clone button padding (#25763)

Changes:

* Rename gt-tl/gt-tc/gt-tr to gt-text-left/gt-text-center/gt-text-right
* The gt-ab and gt-br-0 are removed because they are not needed anymore
* Fix the clone dropdown button padding by ":not(.icon)"

Before:

<details>


![image](https://github.com/go-gitea/gitea/assets/2114189/eb030633-622c-4ca7-8e88-ce010d9f51a6)

</details>

After:

<details>


![image](https://github.com/go-gitea/gitea/assets/2114189/64c09403-bf21-439c-88f1-780b34ccab6b)

</details>

Fixes #25758

Co-authored-by: Giteabot <teabot@gitea.io>

templates/repo/migrate/migrate.tmpl

@@ -16,10 +16,10 @@
 							{{svg (printf "gitea-%s" .Name) 184}}
 						{{end}}
 						<div class="content">
-							<div class="header gt-tc">
+							<div class="header gt-text-center">
 								{{.Title}}
 							</div>
-							<div class="description gt-tc">
+							<div class="description gt-text-center">
 								{{(printf "repo.migrate.%s.description" .Name) | $.locale.Tr}}
 							</div>
 						</div>