Migrate to C++ 20 & switch to websocketpp develop branch (#4252)

* feat: c++ 20

* fix: c++ 20 deprecations

* fix(msvc): warnings

* chore: add changelog entry

* fix: formatting

* Update websocketpp to the `develop` branch

* Specify other template type in FlagsEnum != operator

* Remove the user of simple template ids in our websocketpp template class

Also standardizes the file a bit by using nested namespaces, using
pragma once

* fix: turn `MAGIC_MESSAGE_SUFFIX` into a `QString`

* hacky unhacky hacky const char hack

Co-authored-by: Rasmus Karlsson <rasmus.karlsson@pajlada.com>

src/CMakeLists.txt

@@ -789,28 +789,32 @@ if (MSVC)
     # Someone adds /W3 before we add /W4.
     # This makes sure, only /W4 is specified.
     string(REPLACE "/W3" "/W4" CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS}")
-    # 4714 - function marked as __forceinline not inlined
-    # 4996 - occurs when the compiler encounters a function or variable that is marked as deprecated.
-    #        These functions may have a different preferred name, may be insecure or have
-    #        a more secure variant, or may be obsolete.
-    # 4505 - unreferenced local version has been removed
-    # 4127 - conditional expression is constant
-    # 4503 - decorated name length exceeded, name was truncated
-    # 4100 - unreferences formal parameter
-    # 4305 - possible truncation of data
-    # 4267 - possible loss of data in return
+    # 4505 - "unreferenced local version has been removed"
+    #        Although this might give hints on dead code,
+    #        there are some cases where it's distracting.
+    #
+    # 4100 - "unreferenced formal parameter"
+    #        There are a lot of functions and methods where
+    #        an argument was given a name but never used.
+    #        There's a clang-tidy rule that will catch this
+    #        for new/updated functions/methods.
+    #
+    # 4267 - "possible loss of data in return"
+    #        These are implicit conversions from size_t to int/qsizetype.
+    #        We don't use size_t in a lot of cases, since
+    #        Qt doesn't use it - it uses int (or qsizetype in Qt6).
     target_compile_options(${LIBRARY_PROJECT} PUBLIC
         /W4
-        # Disable the following warnings
-        /wd4714
-        /wd4996
+        # 5038 - warnings about initialization order
+        /w15038
+        # 4855 - implicit capture of 'this' via '[=]' is deprecated
+        /w14855
+        # Disable the following warnings (see reasoning above)
         /wd4505
-        /wd4127
-        /wd4503
         /wd4100
-        /wd4305
         /wd4267
     )
+    target_compile_definitions(${LIBRARY_PROJECT} PUBLIC NOMINMAX)
 else ()
     target_compile_options(${LIBRARY_PROJECT} PUBLIC
         -Wall