Suppress a new Lint/MixedCaseRange offense

Follow up rubocop/rubocop#11561.

```console
$ bundle exec rake
(snip)

Offenses:

lib/rubocop/cop/rails/time_zone.rb:58:33: W: [Correctable] Lint/MixedCaseRange: Ranges from upper to lower case
ASCII letters may include unintended characters. Instead of A-z (which also includes several symbols)
specify each range individually: A-Za-z and individually specify any symbols.
        TIMEZONE_SPECIFIER = /([A-z]|[+-]\d{2}:?\d{2})\z/.freeze
                                ^^^

281 files inspected, 1 offense detected, 1 offense autocorrectable
```

Author: koic

lib/rubocop/cop/rails/time_zone.rb

@@ -55,7 +55,7 @@ class TimeZone < Base
 
         ACCEPTED_METHODS = %i[in_time_zone utc getlocal xmlschema iso8601 jisx0301 rfc3339 httpdate to_i to_f].freeze
 
-        TIMEZONE_SPECIFIER = /([A-z]|[+-]\d{2}:?\d{2})\z/.freeze
+        TIMEZONE_SPECIFIER = /([A-Za-z]|[+-]\d{2}:?\d{2})\z/.freeze
 
         def on_const(node)
           mod, klass = *node