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

tests/SelfTest/IntrospectiveTests/String.tests.cpp

@@ -177,7 +177,7 @@ TEST_CASE("StringRef at compilation time", "[Strings][StringRef][constexpr]") {
         STATIC_REQUIRE_FALSE(sr1.empty());
         STATIC_REQUIRE(sr1.size() == 3);
 
-        using Catch::operator"" _sr;
+        using Catch::operator""_sr;
         constexpr auto sr2 = ""_sr;
         STATIC_REQUIRE(sr2.empty());
         STATIC_REQUIRE(sr2.size() == 0);