Rails deprecation warnings

[xmunoz]

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• What are we trying to do?
  Eliminate Rails deprecation warnings, so we aren't caught off guard with Rails configuration options are completely removed.

• What feature or behavior is this required for?
  Keeping things up to date.

• How could we solve this issue? (Not knowing is okay!)
  Here are the deprecation warnings when the test suite runs:

DEPRECATION WARNING: `Rails.application.secrets` is deprecated in favor of `Rails.application.credentials` and will be removed in Rails 7.2. (called from <main> at /home/runner/work/dcaf_case_management/dcaf_case_management/config/environment.rb:5)
DEPRECATION WARNING: Setting action_dispatch.show_exceptions to false is deprecated. Set to :none instead. (called from block (4 levels) in <class:LinesControllerTest> at /home/runner/work/dcaf_case_management/dcaf_case_management/test/controllers/lines_controller_test.rb:34)

• Anything else?
  Nope, should be pretty straight forward.