minor #5421 Update the name of the branch for new BC features (Restless-ET)

This PR was merged into the 2.3 branch.

Discussion
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Update the name of the branch for new BC features

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.3+
| Fixed tickets | -

Updated section ```Choose the right Branch``` to refer branch ```2.8``` as the one for new BC features.

PS- Will this be merged into other the branches or should I create a PR for the other ones too.

Commits
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ac0a90a Update branch for new BC features

Author: xabbuh

contributing/code/patches.rst

@@ -110,7 +110,7 @@ work:
 * ``2.3``, if you are fixing a bug for an existing feature (you may have
   to choose a higher branch if the feature you are fixing was introduced
   in a later version);
-* ``2.7``, if you are adding a new feature which is backward compatible;
+* ``2.8``, if you are adding a new feature which is backward compatible;
 * ``master``, if you are adding a new and backward incompatible feature.
 
 .. note::