Add 6.8 to the remote clusters compatibility table (#42389)

The table does not include 6.8 as it was written before we knew we were releasing it. This commit adds it.

docs/reference/modules/remote-clusters.asciidoc

@@ -30,24 +30,25 @@ node to the selected remote _gateway nodes_ only.
 The _gateway nodes_ selection depends on the following criteria:
 
 - *version*: Remote nodes must be compatible with the cluster they are
-registered to. This is subject to the same rules as <<rolling-upgrades>>.
-Any node can communicate with any other node on the same major version (e.g.
-6.0 can talk to any 6.x node). Only nodes on the last minor version of a
-certain major version can communicate with nodes on the following major
-version (e.g. 6.7 can communicate with 7.0, as well as any 7.x node, while
- 6.6 or earlier cannot talk to any 7.x node). Note that version compatibility
- is symmetric, meaning that if 6.7 can communicate with 7.0, 7.0 can also
- communicate with 6.7. The matrix below summarizes compatibility as described
- above.
+registered to. This is subject to rules that are similar to those for
+<<rolling-upgrades>>. Any node can communicate with any other node on the same
+major version (e.g. 7.0 can talk to any 7.x node). Only nodes on the last minor
+version of a certain major version can communicate with nodes on the following
+major version. Note that in the 6.x series, 6.8 can communicate with any 7.x
+node, while 6.7 can only communicate with 7.0. Version compatibility  is
+symmetric, meaning that if 6.7 can communicate with 7.0, 7.0 can also
+communicate with 6.7. The matrix below summarizes compatibility as described above.
 
 [cols="^,^,^,^,^,^"]
 |====
-| Compatibility | 5.0->5.5 | 5.6 | 6.0->6.6 | 6.7 | 7.x
-| 5.0->5.5      |    Yes   | Yes |    No    | No  | No
-| 5.6           |    Yes   | Yes |    Yes   | Yes | No
-| 6.0->6.6      |    No    | Yes |    Yes   | Yes | No
-| 6.7           |    No    | Yes |    Yes   | Yes | Yes
-| 7.x           |    No    | No  |    No    | Yes | Yes
+| Compatibility | 5.0->5.5 | 5.6 | 6.0->6.6 | 6.7 | 6.8 | 7.0 | 7.1->7.x
+| 5.0->5.5      |    Yes   | Yes |    No    | No  | No  | No  |    No
+| 5.6           |    Yes   | Yes |    Yes   | Yes | Yes | No  |    No
+| 6.0->6.6      |    No    | Yes |    Yes   | Yes | Yes | No  |    No
+| 6.7           |    No    | Yes |    Yes   | Yes | Yes | Yes |    No
+| 6.8           |    No    | Yes |    Yes   | Yes | Yes | Yes |    Yes
+| 7.0           |    No    | No  |    No    | Yes | Yes | Yes |    Yes
+| 7.1->7.x      |    No    | No  |    No    | No  | Yes | Yes |    Yes
 |====
 
 - *role*: Dedicated master nodes never get selected.