Bump version to 1.21-dev (#1661)

Author: alcaeus

phongo_version.h

@@ -22,9 +22,9 @@
  * publishing a release. */
 
 /* clang-format off */
-#define PHP_MONGODB_VERSION "1.20.0dev"
+#define PHP_MONGODB_VERSION "1.21.0dev"
 #define PHP_MONGODB_STABILITY "devel"
-#define PHP_MONGODB_VERSION_DESC 1,20,0,0
+#define PHP_MONGODB_VERSION_DESC 1,21,0,0
 /* clang-format on */
 
 #endif /* PHONGO_VERSION_H */