Improve okta_email_customization resource

[monde]

• Correct delete in okta_email_customization resource
• Improve it's documentation
• Deprecate force_is_default and no-op its behavior
• Note in documentation to use depends_on meta argument to manage primary is_default and secondary dependent customizations.

/cc @BalaGanaparthi

go clean -testcache && TF_ACC=1 go test -tags unit -mod=readonly -test.v -run ^TestAccResourceOktaEmailCustomization_crud$ ./okta 2>&1
2023/06/29 11:39:20 [INFO]  running with default http client
=== RUN   TestAccResourceOktaEmailCustomization_crud
2023/06/29 11:39:34 [INFO]  running with default http client
2023/06/29 11:39:34 [INFO]  running with experimental max_api_capacity configuration at 80%
2023/06/29 11:39:34 [INFO]  running with default http client
2023/06/29 11:39:34 [INFO]  running with experimental max_api_capacity configuration at 80%
--- PASS: TestAccResourceOktaEmailCustomization_crud (14.20s)
PASS
ok      github.com/okta/terraform-provider-okta/okta    14.963s