Set GDB version number to 15.1.

This commit changes gdb/version.in to 15.1.
diff --git a/gdb/version.in b/gdb/version.in
index 19415a5..adbc6d2 100644
--- a/gdb/version.in
+++ b/gdb/version.in
@@ -1 +1 @@
-15.0.91.DATE-git
+15.1