Some timezones like `Europe/London` have multiple mappings in
`ActiveSupport::TimeZone::MAPPING` so return all of them instead
of the first one found by using `Hash#value`. e.g:

  # Before
  ActiveSupport::TimeZone.country_zones("GB") # => ["Edinburgh"]

  # After
  ActiveSupport::TimeZone.country_zones("GB") # => ["Edinburgh", "London"]

Fixes #31668.

(cherry picked from commit 2d95956e)