Add 4.2.Z back to the maintenance list

Some Rails core members agreed to continue to maintain 4.2 until 5.1 is
released.

[ci skip]

guides/source/maintenance_policy.md

@@ -44,7 +44,7 @@ from.
 In special situations, where someone from the Core Team agrees to support more series,
 they are included in the list of supported series.
 
-**Currently included series:** `5.0.Z`.
+**Currently included series:** `5.0.Z`, `4.2.Z`.
 
 Security Issues
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