Remove wxUSE_STL which is not really used any longer

wxString is always based on std::[w]string since 2c0c727f49 (Remove wx
own wxStringImpl implementation, 2022-11-16) and all containers use
standard containers by default too now -- and there is a separate
wxUSE_STD_CONTAINERS for this anyhow.

The only remaining use of wxUSE_STL was as the default value for
wxUSE_STD_STRING_CONV_IN_WXSTRING option, but it's not really needed
for this neither, and this option can just be set to 0 by default.

Also add wxUSE_CHAR_CONV_IN_WXSTRING which can now be set to 0 too to
disable all unwanted implicit conversions (even "safe" ones, to wide
strings, in addition to the unsafe ones to narrow strings that could be
already disabled with wxUSE_UNSAFE_WXSTRING_CONV) to allow people who
don't want to have any implicit conversions at all to do it.

Keep --enable-stl configure option for compatibility, but warn if it is
used to tell people that it is not needed any longer.

docs/changes.txt

@@ -92,6 +92,11 @@ Changes in behaviour which may result in build errors
   the existing code. Alternatively, explicitly set wxUSE_STD_CONTAINERS=0 when
   compiling the library to preserve full compatibility with the old versions.
 
+- wxUSE_STL option doesn't exist any longer, standard library is always used.
+  However previously setting wxUSE_STL=1 enabled implicit conversion from
+  wxString to std::[w]string which are not enabled by default now, please set
+  wxUSE_STD_STRING_CONV_IN_WXSTRING=1 explicitly if you need them.
+
 - wxMotif and wxGTK1 ports have been removed, please use wxWidgets 3.2 if you
   still need them.
 

docs/doxygen/mainpages/const_cpp.h

@@ -339,12 +339,11 @@ more details.
          with old wxWidgets versions. Changing it is not recommended.}
 @itemdef{wxUSE_UNSAFE_WXSTRING_CONV,
          this option determines if unsafe implicit conversions of wxString to
-         @c char* or @c std::string (depending on whether @c wxUSE_STL is 0 or
-         1) are defined. It is set to 1 by default for compatibility reasons,
-         however it is recommended to set it to 0 for the new projects. See
-         also @c wxNO_UNSAFE_WXSTRING_CONV below for an alternative way of
-         disabling these unsafe conversions not requiring rebuilding the
-         library.}
+         @c char* or @c std::string (depending on whether
+         @c wxUSE_STD_STRING_CONV_IN_WXSTRING is 0 or 1) are defined.
+         It is set to 1 by default for compatibility reasons, but you may set
+         @c wxNO_UNSAFE_WXSTRING_CONV described below to disable these unsafe
+         unsafe conversions without rebuilding the library.}
 @endDefList
 
 @section page_cppconst_miscellaneous Miscellaneous