Fix encoding with libxml2 >= 2.12 push parser

[lenz1111]

The enc parameter of htmlCreatePushParserCtxt() was deprecated and
is ignored in libxml2 >= 2.12.0. As a result, the HTML push parser defaults
to ISO-8859-1 encoding regardless of the value passed, causing UTF-8 multibyte
characters to be misinterpreted as Latin-1.

This breaks SOGo on systems with libxml2 >= 2.12 (e.g., RHEL 10):

• Viewing HTML emails with non-ASCII characters shows garbled text
• Plain text alternative parts of sent emails are double-encoded

Fix by calling xmlSwitchEncoding() after creating the push parser
context. This is the documented replacement for the deprecated enc
parameter per the libxml2 2.12.0 NEWS:

"Calling xmlSwitchEncoding from client code is now fully supported,
for example to override the encoding for the push parser."

The fix is backward-compatible with libxml2 2.9.x where it is a
harmless no-op since the encoding was already set via the enc
parameter.