Use mask-based fade-out effect for .new-menu (#27181)

The `.new-menu` was using a pseudo-element based fade-out effect.
Replace this with a more modern mask-based effect which in this case
required a child element to avoid fading out the background as well, so
I applied it to child `new-menu-inner` which was present on all these
menus except explore where I added it.

There is no visual difference except that the items on the explore page
have no `gap` between them any longer, making it consistent with other
menus. Before and after:

<img width="221" alt="Screenshot 2023-09-21 at 21 13 19"
src="b4a38ce2-cee1-4c54-84a5-e1d0bfd79e29">
<img width="222" alt="Screenshot 2023-09-21 at 21 32 36"
src="bb6b1335-d935-4ad4-bb85-3b0fc3027c2b">

Also, this cleans up the related CSS vars:

- `--color-header-wrapper-transparent` is removed, no longer needed
- `--color-header-wrapper` is defined in base theme as well, was
previously unset and therefor transparent.

[no whitespace
diff](https://github.com/go-gitea/gitea/pull/27181/files?diff=unified&w=1)
[demo of mask fade](https://jsfiddle.net/silverwind/tsfadb3u/)

web_src/css/themes/theme-arc-green.css

@@ -198,7 +198,6 @@
   --color-input-border: var(--color-secondary);
   --color-input-border-hover: var(--color-secondary-dark-1);
   --color-header-wrapper: #202430;
-  --color-header-wrapper-transparent: #20243000;
   --color-light: #00000028;
   --color-light-mimic-enabled: rgba(0, 0, 0, calc(40 / 255 * 222 / 255 / var(--opacity-disabled)));
   --color-light-border: #ffffff28;