Ruby: add change note for system command execution sink bug

Author: nickrolfe

ruby/ql/lib/change-notes/2022-07-11-command-execution.md

@@ -0,0 +1,6 @@
+---
+category: minorAnalysis
+---
+* Fixed a bug causing every expression in the database to be a considered a system-command execution sink when calls to any of the following methods exist:
+  * The `spawn", "fspawn", "popen4", "pspawn", "system", "_pspawn" methods and the backtick operator from the `POSIX::spawn` gem.
+  * The `execute_command`, `rake`, `rails_command`, and `git` methods in `Rails::Generation::Actions`.