Ruby: add changenotes for regexp constant value changes

Author: nickrolfe

ruby/ql/lib/change-notes/2022-03-25-deprecate-getStringOrSymbol.md

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+---
+ category: minorAnalysis
+---
+ *  Whereas `ConstantValue::getString()` previously returned both string and regular-expression values, it now returns only string values. The same applies to `ConstantValue::isString(value)`.
+ *  Regular-expression values can now be accessed with the new predicates `ConstantValue::getRegExp()`, `ConstantValue::isRegExp(value)`, and `ConstantValue::isRegExpWithFlags(value, flags).

ruby/ql/lib/change-notes/2022-03-25-regexp-constant-values.md

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+---
+ category: deprecated
+---
+ * `ConstantValue::getStringOrSymbol` and `ConstantValue::isStringOrSymbol`, which return/hold for all string-like values (strings, symbols, and regular expressions), have been renamed to `ConstantValue::getStringlikeValue` and `ConstantValue::isStringlikeValue`, respectively. The old names have been marked as `deprecated`.