contributing: Align backporting text and example (#2978)

The first part of the paragraph talks about backporting to all branches in between main and furthest backport. However, the example pushed the idea of backport to the furthest instead. The new example is now only about backporting to the in-between branch while the paragraph addresses the backport in general and packport to the closest branch first.

The new example uses version numbers with the major version 3 because it is only an example, so it does not need to be updated.

doc/development/submitting/general.md

@@ -147,11 +147,11 @@ available), and compilation of the whole source tree (ideally after the
 the original commit to "main". Also note that not all these steps have to be
 done manually, you can take an advantage of our [​CI](https://github.com/OSGeo/grass/actions).
 
-Often there is more than one active release branch, if you are backporting, make
-sure you always backport to all the branches between "main" and the furthest
-branch you are backporting to. For example, let's say we have "main" and branches
-8.2 and 7.8, if you backport to 8.2, you should also consider to backport to 7.8.
-You can also choose to backport only to the closest branch, in our example 8.2.
+Often there is more than one active release branch. You can also choose to backport
+only to the closest branch. If you are backporting to other release branches than
+just the closets one, make sure you always backport to all the branches between "main"
+and the furthest branch you are backporting to. For example, let's say we have
+release branches 3.6 and 3.5, if you backport to 3.5, you should backport to 3.6, too.
 
 Backport only complete fixes. When you are not sure if the fix is complete or if
 there is an possibility that some details such as wording will change, wait with