Fix additional margin in commit message

Since adding the `formatted-text-endpoint` plugin endpoint in
change 437561 the `<pre class="plaintext">` may not be the `last-child`
anymore. So the css rule for setting `margin: 0` needs to be
adapted.

Release-Notes: skip
Google-Bug-Id: b/377535764
Change-Id: I16d132198c2b08678b7765a6f729e9cef233e62a
(cherry picked from commit 74535522882af0133d85f9e718dc64cd210713fb)
diff --git a/polygerrit-ui/app/elements/shared/gr-formatted-text/gr-formatted-text.ts b/polygerrit-ui/app/elements/shared/gr-formatted-text/gr-formatted-text.ts
index 900e9ed..fa2a4ff 100644
--- a/polygerrit-ui/app/elements/shared/gr-formatted-text/gr-formatted-text.ts
+++ b/polygerrit-ui/app/elements/shared/gr-formatted-text/gr-formatted-text.ts
@@ -68,7 +68,8 @@
         p:last-child,
         ul:last-child,
         blockquote:last-child,
-        pre:last-child {
+        pre:last-child,
+        pre.plaintext {
           margin: 0;
         }
         blockquote {