Update changelog for '-py3.7' and 'referencing'

Author: sirosen

CHANGELOG.rst

@@ -11,6 +11,15 @@ Unreleased
 .. vendor-insert-here
 
 - Update vendored schemas (2023-07-18)
+- Remove support for python3.7
+- The minimum supported version of the `jsonschema` library is now `4.18.0`,
+  which introduces new `$ref` resolution behavior and fixes. That behavior is
+  used in all cases, which should result in faster evaluation especially on
+  large schemas.
+- `$ref` usage may now refer to YAML, TOML, or JSON5 files, or any other
+  non-JSON format supported by `check-jsonschema`. The file type is inferred
+  only from the file extension in these cases and defaults to JSON if there is
+  no recognizable extension.
 
 0.23.3
 ------