Prior to this commit, the JAXB message converters would only rely on the
encoding declaration inside the XML document for reading the document.
This would then use the default UTF-8 encoding, even if the HTTP message
has the `"application/xml;charset=iso-8859-1"` Content-Type.
This commit ensures that both `Jaxb2CollectionHttpMessageConverter` and
`Jaxb2RootElementHttpMessageConverter` use the encoding declared in the
HTTP Content-Type, if present.
Fixes gh-34745
2af0323c21