feat!: Switch to method+handler Datadog resource name format

[timmc-edx]

Replaces all instances of DD_DJANGO_USE_LEGACY_RESOURCE_FORMAT with DD_DJANGO_USE_HANDLER_RESOURCE_FORMAT. Now resource names will be prefixed with the HTTP method, like
GET lms.djangoapps.branding.views.index.

This is not ideal and loses us some precision in the resource name but gives us something more readable, writable, and pronounceable. See edx/edx-arch-experiments#620 for more background.

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Make sure that the following steps are done before merging:

• Have a Site Reliability Engineer review the PR if you don't own all of the services impacted.
• If you are adding any new default values that need to be overridden when this change goes live, update internal repos and add an entry to the top of the CHANGELOG.
• Performed the appropriate testing.