Cleanup 8.17 to 9.0 doc with review suggestions

Fix redundant wording.
Clean up code blocks and links.

doc/update/8.17-to-9.0.md

@@ -62,15 +62,15 @@ from source at the nodejs.org website.
 <https://nodejs.org/en/download/>
 
 
-Since GitLab 8.17, GitLab requires the use of yarn yarn >= v0.17.0  to manage
-javascript dependencies.
+Since 8.17, GitLab requires the use of yarn `>= v0.17.0` to manage
+JavaScript dependencies.
 
-    # install yarn
-    curl --location https://yarnpkg.com/install.sh | bash -
-
-More information can be found on the yarnpkg.com website.
+```bash
+# install yarn
+curl --location https://yarnpkg.com/install.sh | bash -
+```
 
-<https://yarnpkg.com/en/docs/install>
+More information can be found on the [yarn website](https://yarnpkg.com/en/docs/install).
 
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