Fix UDL definitions for C++23

Technically, the declaration should not have a space between
the quotes and the underscore, because `_foo` is a reserved
identifier, but `""_foo` is not. In general it works, but newer
Clang versions warn about this, because WG21 wants to deprecate
and later remove this form completely.

src/catch2/catch_approx.cpp

@@ -70,10 +70,10 @@ namespace Catch {
     }
 
 namespace literals {
-    Approx operator "" _a(long double val) {
+    Approx operator ""_a(long double val) {
         return Approx(val);
     }
-    Approx operator "" _a(unsigned long long val) {
+    Approx operator ""_a(unsigned long long val) {
         return Approx(val);
     }
 } // end namespace literals