Bug#35781622 Switch from -std=c++17 to -std=c++20

Post-push fix: -Wambiguous-reversed-operator

Clang 16 on windows says:
src\user_map.cc(186,35):
error: ISO C++20 considers use of overloaded operator '=='
(with operand types 'Sid' and 'Sid')
to be ambiguous despite there being a unique best viable function
[-Werror,-Wambiguous-reversed-operator]
    if (p->m_sid.is_user() && who == p->m_sid) {
                              ~~~ ^  ~~~~~~~~

Change-Id: Ib0fe9c739b6466763ec617890e9320b7c211c44a

Author: Tor Didriksen

libmysql/authentication_win/common.cc

@@ -272,7 +272,7 @@ const char *Sid::as_string() {
 
 #endif
 
-bool Sid::operator==(const Sid &other) {
+bool Sid::operator==(const Sid &other) const {
   if (!is_valid() || !other.is_valid()) return false;
 
   return EqualSid(m_data->User.Sid, other.m_data->User.Sid);

libmysql/authentication_win/common.h

@@ -233,7 +233,7 @@ class Sid {
 
   bool is_user(void) const { return m_type == SidTypeUser; }
 
-  bool operator==(const Sid &);
+  bool operator==(const Sid &) const;
 
   operator PSID() const { return (PSID)m_data->User.Sid; }