Resolve "Ensure diff collection limits are configurable" [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

➜  gitlab git:(31063-ensure-diff-collection-limits-are-monitorable) ✗ rake db:migrate:up VERSION=20210527134019
== 20210527134019 AddDiffMaxFilesToApplicationSettings: migrating =============
-- add_column(:application_settings, :diff_max_files, :integer, {:default=>1000, :null=>false})
   -> 0.0072s
== 20210527134019 AddDiffMaxFilesToApplicationSettings: migrated (0.0073s) ====

➜  gitlab git:(31063-ensure-diff-collection-limits-are-monitorable) ✗ rake db:migrate:down VERSION=20210527134019
== 20210527134019 AddDiffMaxFilesToApplicationSettings: reverting =============
-- remove_column(:application_settings, :diff_max_files, :integer, {:default=>1000, :null=>false})
   -> 0.0035s
== 20210527134019 AddDiffMaxFilesToApplicationSettings: reverted (0.0068s) ====

➜  gitlab git:(31063-ensure-diff-collection-limits-are-monitorable) ✗ rake db:migrate:down VERSION=20210527133919
== 20210527133919 AddDiffMaxLinesToApplicationSettings: reverting =============
-- remove_column(:application_settings, :diff_max_lines, :integer, {:default=>50000, :null=>false})
   -> 0.0037s
== 20210527133919 AddDiffMaxLinesToApplicationSettings: reverted (0.0067s) ====

➜  gitlab git:(31063-ensure-diff-collection-limits-are-monitorable) ✗ rake db:migrate:up VERSION=20210527133919
== 20210527133919 AddDiffMaxLinesToApplicationSettings: migrating =============
-- add_column(:application_settings, :diff_max_lines, :integer, {:default=>50000, :null=>false})
   -> 0.0049s
== 20210527133919 AddDiffMaxLinesToApplicationSettings: migrated (0.0050s) ====

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Does this MR meet the acceptance criteria?

Conformity

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Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
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Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
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Related to #31063 (closed)