[release doc] Add a paragraph for label maintenance (#8422)

Refs #8420

Co-authored-by: Jacob Walls <jacobtylerwalls@gmail.com>

Author: Pierre-Sassoulas

doc/development_guide/contributor_guide/release.md

@@ -45,6 +45,10 @@ branch
   `maintenance/2.3*`.). There's a lot of configuration done in these settings, do NOT
   recreate it from scratch.
 - Delete the `maintenance/X.Y-1.x` branch. (For example: `maintenance/2.3.x`)
+- Select all the issues labelled `backport maintenance/X.Y-1.x` and label them
+  `backported`, then rename the `backport maintenance/X.Y-1.x` label to
+  `backport maintenance/X.Y.x` (for example rename `backport maintenance/2.3.x` to
+  `backport maintenance/2.4.x`)
 - Close the current milestone and create the new ones (For example: close `2.4.0`,
   create `2.4.1` and `2.6.0`)
 - Hide and deactivate all the patch releases for the previous minor release on