immutable_cache: Changelog

Version Message
6.1.0 - Extend minimum supported php version from php 8.0 to 7.2
- Work around bug adding '-lpthread' as a shared library dependency in php before 7.4.14. Related to
- Switch default for immutable_cache.serializer to "default" (store the php arrays and strings and scalars in shared memory rather than serialization when added value is an array)