doc: update backport labels in release guide

Add `backport-open-v1.x,backported-to-v1.x` labels to `branch-diff`
commands as these indicate pull requests that are being manually
backported and should not be cherry-picked.

PR-URL: #39544
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Beth Griggs <bgriggs@redhat.com>
Reviewed-By: Danielle Adams <adamzdanielle@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

Author: richardlau

doc/guides/releases.md

@@ -175,7 +175,7 @@ duplicate or not.
 For a list of commits that could be landed in a patch release on v1.x:
 
 ```console
-$ branch-diff v1.x-staging master --exclude-label=semver-major,semver-minor,dont-land-on-v1.x,backport-requested-v1.x,backport-blocked-v1.x --filter-release --format=simple
+$ branch-diff v1.x-staging master --exclude-label=semver-major,semver-minor,dont-land-on-v1.x,backport-requested-v1.x,backport-blocked-v1.x,backport-open-v1.x,backported-to-v1.x --filter-release --format=simple
 ```
 
 Previously released commits and version bumps do not need to be
@@ -195,7 +195,7 @@ command. (For semver-minor releases, make sure to remove the `semver-minor` tag
 from `exclude-label`.)
 
 ```console
-$ branch-diff v1.x-staging master --exclude-label=semver-major,semver-minor,dont-land-on-v1.x,backport-requested-v1.x,backport-blocked-v1.x --filter-release --format=sha --reverse | xargs git cherry-pick
+$ branch-diff v1.x-staging master --exclude-label=semver-major,semver-minor,dont-land-on-v1.x,backport-requested-v1.x,backport-blocked-v1.x,backport-open-v1.x,backported-to-v1.x --filter-release --format=sha --reverse | xargs git cherry-pick
 ```
 
 When cherry-picking commits, if there are simple conflicts you can resolve