doc: remove instructions for unsupported Node.js versions

Remove special V8 maintenance instructions for versions of Node.js prior
to 9.0.0. Perform other minor edits in nearby text.

PR-URL: #39185
Reviewed-By: Richard Lau <rlau@redhat.com>
Reviewed-By: Michael Dawson <midawson@redhat.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Gerhard Stöbich <deb2001-github@yahoo.de>

doc/guides/maintaining-V8.md

@@ -213,17 +213,12 @@ to be cherry-picked in the Node.js repository and V8-CI must test the change.
   * Checkout a branch off the appropriate *vY.x-staging* branch (e.g.
     *v6.x-staging* to fix an issue in V8 5.1).
   * Cherry-pick the commit(s) from the V8 repository.
-  * On Node.js < 9.0.0: Increase the patch level version in `v8-version.h`.
-    This will not cause any problems with versioning because V8 will not
-    publish other patches for this branch, so Node.js can effectively bump the
-    patch version.
-  * On Node.js >= 9.0.0: Increase the `v8_embedder_string` number in
-    `common.gypi`.
+  * Increase the `v8_embedder_string` number in `common.gypi`.
   * In some cases the patch may require extra effort to merge in case V8 has
-    changed substantially. For important issues we may be able to lean on the
+    changed substantially. For important issues, we may be able to lean on the
     V8 team to get help with reimplementing the patch.
-  * Open a cherry-pick PR on `nodejs/node` targeting the *vY.x-staging* branch
-    and notify the `@nodejs/v8` team.
+  * Open a cherry-pick pull request on `nodejs/node` targeting the
+    *vY.x-staging* branch and notify the `@nodejs/v8` team.
   * Run the Node.js [V8 CI][] in addition to the [Node.js CI][].
     The CI uses the `test-v8` target in the `Makefile`, which uses
     `tools/make-v8.sh` to reconstruct a git tree in the `deps/v8` directory to