Ruby: add change note

Author: asgerf

ruby/ql/lib/change-notes/2023-06-28-tracking-on-demand.md

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+---
+category: library
+---
+* The API graph library (`codeql.ruby.ApiGraphs`) has been significantly improved, with better support for inheritance,
+  and data-flow nodes can now be converted to API nodes by calling `.track()` or `.backtrack()` on the node.
+  API graphs allow for efficient modelling of how a given value is used by the code base, or how values produced by the code base
+  are consumed by a library. See the documentation for `API::Node` for details and examples.