Set GDB version number to 12.1.

This commit changes gdb/version.in to 12.1.
diff --git a/gdb/version.in b/gdb/version.in
index 02cb558..40e6bd9 100644
--- a/gdb/version.in
+++ b/gdb/version.in
@@ -1 +1 @@
-12.0.90.DATE-git
+12.1