Drop public access to CodecRegistryConstants.PRIMITIVE_CODECS #908

CodecRegistryConstants.PRIMITIVE_CODECS is a public mutable array kept for source and binary compatibility.

Follow-up for 4.20.0:

• keep the safer accessor introduced by Drop Java 8 support #900
• remove or narrow direct public access to PRIMITIVE_CODECS
• update downstream call sites and migration notes as needed

Related to #900.