Fix issue that sync_on_commit is not set (#21309)

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Fixes #21308.

With the original frontend template
`templates/repo/settings/options.tmpl`, the field
`push_mirror_sync_on_commit` is always empty even when checkbox is
checked. Removing `value` from the input tag seems to solve the issue,
and will set `push_mirror_sync_on_commit: on` when the checkbox is
checked.

(I'm not familiar with the frontend logics Gitea is using, so I don't
really understand the cause of it)

Author: harryzcy

templates/repo/settings/options.tmpl

@@ -221,7 +221,7 @@
 										</details>
 										<div class="field">
 											<div class="ui checkbox">
-												<input id="push_mirror_sync_on_commit" name="push_mirror_sync_on_commit" type="checkbox" value="{{.push_mirror_sync_on_commit}}">
+												<input id="push_mirror_sync_on_commit" name="push_mirror_sync_on_commit" type="checkbox" {{if .push_mirror_sync_on_commit}}checked{{end}}>
 												<label for="push_mirror_sync_on_commit">{{.locale.Tr "repo.mirror_sync_on_commit"}}</label>
 											</div>
 										</div>