Set GDB version number to 13.1.

This commit changes gdb/version.in to 13.1.
diff --git a/gdb/version.in b/gdb/version.in
index 149ed94..cdb4ac4 100644
--- a/gdb/version.in
+++ b/gdb/version.in
@@ -1 +1 @@
-13.0.91.DATE-git
+13.1