Replace ExcludedMethods by IgnoredMethods

`ExcludedMethods` becomes obsolete in later version
of rubocop, showing warnings.

See rubocop/rubocop#9098
and https://github.com/rubocop/rubocop/blob/1e55b1aa5e4c5eaeccad5d61f08b7930ed6bc341/relnotes/v1.5.0.md

.rubocop.yml

@@ -42,7 +42,7 @@ Metrics/MethodLength:
 
 Metrics/BlockLength:
   Enabled: true
-  ExcludedMethods: ['describe', 'context', 'define', 'factory', 'namespace']
+  IgnoredMethods: ['describe', 'context', 'define', 'factory', 'namespace']
 
 Metrics/AbcSize:
   Enabled: true