fix: Add FlushSoftwareUpdatePlans to jamfproapi_enrollment_settings.go

This should ensure proper state detection of `flush_software_update_plans` and stop the `jamfpro_user_initiated_enrollment_settings` resource trying to update on every run despite already being set to `true`.

Otherwise we see the following on every `terraform plan` or `terraform apply` run.

```
  # module.settings-userinitiatedenrollment.jamfpro_user_initiated_enrollment_settings.user_initiated_enrollment_settings will be updated in-place
  ~ resource "jamfpro_user_initiated_enrollment_settings" "user_initiated_enrollment_settings" {
      ~ flush_software_update_plans                     = false -> true
        id                                              = "jamfpro_user_initiated_enrollment_settings_singleton"
        # (8 unchanged attributes hidden)

        # (3 unchanged blocks hidden)
    }
```

Author: neilmartin83

sdk/jamfpro/jamfproapi_enrollment_settings.go

@@ -28,6 +28,7 @@ type EnrollmentSubsetCertificateSettings struct {
 	FlushLocationHistoryInformation     bool                               `json:"flushLocationHistoryInformation"`
 	FlushPolicyHistory                  bool                               `json:"flushPolicyHistory"`
 	FlushExtensionAttributes            bool                               `json:"flushExtensionAttributes"`
+	FlushSoftwareUpdatePlans            bool                               `json:"flushSoftwareUpdatePlans"`
 	FlushMdmCommandsOnReenroll          string                             `json:"flushMdmCommandsOnReenroll"`
 	MacOsEnterpriseEnrollmentEnabled    bool                               `json:"macOsEnterpriseEnrollmentEnabled"`
 	ManagementUsername                  string                             `json:"managementUsername"`