Release 6.22.0: Added configuration option whether to prefer GSSAPI authentication mechanism if supported by SIEVE backend

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Added configuration option to file 'mailfilter.properties' whether to prefer GSSAPI authentication mechanism if supported by SIEVE backend

# Specify whether to prefer GSSAPI authentication mechanism if supported by SIEVE backend
# Default is false.
com.openexchange.mail.filter.preferGSSAPI=false