enhance: update mermaid to v12 (#39331)

Update to [mermaid
v12](https://github.com/mermaid-js/mermaid/releases#release-mermaid@12.0.0),
ELK is now the default layout so the plugin and all supporting code is
gone.

Layout switching to previous `dagre` layout via frontmatter works as
expected. Diagrams use the new `neo` default look which renders them
slightly different (smaller boxes) then the `classic` look that was
default in v11.

The `neo` look got some tweaks to remove shadows and gradient to look
better. Also did some related refactors and mermaid dragging now works
on touch devices too.

web_src/css/markup/content.css

@@ -427,7 +427,7 @@ html[data-gitea-theme-dark="false"] .markup img[src*="#gh-dark-mode-only"] {
   display: block;
   border: none;
   width: 100%;
-  height: var(--height-loading); /* actual height is set in JS after loading */
+  height: var(--height-loading); /* actual height is set in JS */
   overflow: hidden;
   color-scheme: normal; /* match the value inside the iframe to allow it to become transparent */
 }

web_src/css/modules/animations.css

@@ -55,6 +55,7 @@ form.single-button-form.is-loading .button {
 .markup pre.is-loading,
 .editor-loading.is-loading {
   height: var(--height-loading);
+  overflow: hidden;
 }
 
 .markup .is-loading > * {