Update diff_max_patch_bytes from 100kb -> 200kb

What does this MR do?

We've increased this value (diff_max_patch_bytes) via the Admin Settings UI for Gitlab.com to 200kb for the past 8 days without significant impact to performance.

Up:

== 20201027211138 IncreaseDefaultDiffMaxPatchBytes: migrating =================
-- change_column_default(:application_settings, :diff_max_patch_bytes, {:from=>102400, :to=>204800})
   -> 0.0412s
== 20201027211138 IncreaseDefaultDiffMaxPatchBytes: migrated (0.0413s) ========
== 20201028204306 MigrateDefaultDiffMaxPatchBytesToMinimum200kb: migrating ====
== 20201028204306 MigrateDefaultDiffMaxPatchBytesToMinimum200kb: migrated (0.0808s)

Down

== 20201028204306 MigrateDefaultDiffMaxPatchBytesToMinimum200kb: reverting ====
== 20201028204306 MigrateDefaultDiffMaxPatchBytesToMinimum200kb: reverted (0.0000s)
== 20201027211138 IncreaseDefaultDiffMaxPatchBytes: reverting =================
-- change_column_default(:application_settings, :diff_max_patch_bytes, {:from=>204800, :to=>102400})
   -> 0.0388s
== 20201027211138 IncreaseDefaultDiffMaxPatchBytes: reverted (0.0431s) ========

Related to #15829 (closed)

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

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
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

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
• Security reports checked/validated by a reviewer from the AppSec team