Improve flex list UI (#26970)

1. There is already `gt-ac`, so no need to introduce `flex-item-center`
2. The `flex-item-baseline` and `.flex-item-icon svg { margin-top: 1px
}` seem to be a tricky patch, they don't resolve the root problem, and
still cause misalignment in some cases.
* The root problem is: the "icon" needs to align with the sibling
"title"
    * So, make the "icon" and the "title" both have the same height 
3. `flex-text-inline` could only be used if the element is really
"inline", otherwise its `vertical-align` would make the box size change.
In most cases, `flex-text-block` is good enough.


![image](https://github.com/go-gitea/gitea/assets/2114189/1b7acfc2-b1c7-4e9c-a983-2fa932026479)

---------

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

templates/shared/variables/variable_list.tmpl

@@ -16,7 +16,7 @@
 	{{if .Variables}}
 	<div class="flex-list">
 		{{range .Variables}}
-		<div class="flex-item flex-item-center">
+		<div class="flex-item gt-ac">
 			<div class="flex-item-leading">
 				{{svg "octicon-pencil" 32}}
 			</div>