Set GDB version number to 16.1.

This commit changes gdb/version.in to 16.1.
diff --git a/gdb/version.in b/gdb/version.in
index 62ce021..c32b0ec 100644
--- a/gdb/version.in
+++ b/gdb/version.in
@@ -1 +1 @@
-16.0.90.DATE-git
+16.1