Fix minor misalignment (#6541)

[skip ci] no relevant tests

It's been there for a while. `flex-start` causes all items to stick to one side and be misaligned if they have different heights.

`align-items` isn't actually needed in this area when the goal is centered vertical alignment, however, unsetting it _could_ break items' height. I couldn't make it do that, but to be safe I kept it and only replaced the value.

## Preview

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There's no difference when they're broken into two rows, on mobile.

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/6541
Reviewed-by: Michael Kriese <michael.kriese@gmx.de>
Reviewed-by: Otto <otto@codeberg.org>
Co-authored-by: 0ko <0ko@noreply.codeberg.org>
Co-committed-by: 0ko <0ko@noreply.codeberg.org>

web_src/css/repo/issue-list.css

@@ -2,7 +2,7 @@
   display: flex;
   flex-wrap: wrap-reverse;
   justify-content: space-between;
-  align-items: flex-start;
+  align-items: center;
   gap: 1rem;
   margin-top: 1rem;
 }