Fix feature test for ABSL_HAVE_STD_OPTIONAL

Fixes #1840

PiperOrigin-RevId: 728197023
Change-Id: I864164fc64512abac19ee6f9dc4e9562e68489a1

Author: derekmauro

absl/base/config.h

@@ -538,7 +538,7 @@ static_assert(ABSL_INTERNAL_INLINE_NAMESPACE_STR[0] != 'h' ||
 // Checks whether C++17 std::optional is available.
 #ifdef ABSL_HAVE_STD_OPTIONAL
 #error "ABSL_HAVE_STD_OPTIONAL cannot be directly set."
-#elif defined(__cpp_lib_optional) && __cpp_lib_optional >= 202106L
+#elif defined(__cpp_lib_optional) && __cpp_lib_optional >= 201606L
 #define ABSL_HAVE_STD_OPTIONAL 1
 #elif defined(ABSL_INTERNAL_CPLUSPLUS_LANG) && \
     ABSL_INTERNAL_CPLUSPLUS_LANG >= 201703L && \