rem: usr: Deprecate the "tkey-gssapi-credential" statement

The :any:`tkey-gssapi-keytab` statement allows GSS-TSIG to be set up in
a simpler and more reliable way than using the
:any:`tkey-gssapi-credential` statement and setting environment
variables (e.g. ``KRB5_KTNAME``). Therefore, the
:any:`tkey-gssapi-credential` statement has been deprecated;
:any:`tkey-gssapi-keytab` should be used instead.

For configurations currently using a combination of both
:any:`tkey-gssapi-keytab` *and* :any:`tkey-gssapi-credential`, the
latter should be dropped and the keytab pointed to by
:any:`tkey-gssapi-keytab` should now only contain the credential
previously specified by :any:`tkey-gssapi-credential`.

See #4204

Merge branch '4204-deprecate-tkey-gssapi-credential' into 'main'

See merge request isc-projects/bind9!10782