feat: v4 to v5 migration for zero_trust_gateway_policy

[tamas-jozsa]

TF_ACC=1 TF_MIGRATE_BINARY_PATH=/Users/tjozsa/cf-repos/sdks/terraform-devstack/tf-migrate/tf-migrate go test -v -run "TestMigrate" ./internal/services/zero_trust_gateway_policy
=== RUN   TestMigrateZeroTrustGatewayPolicy_V4ToV5_Minimal
--- PASS: TestMigrateZeroTrustGatewayPolicy_V4ToV5_Minimal (22.94s)
=== RUN   TestMigrateZeroTrustGatewayPolicy_V4ToV5_WithRuleSettings
--- PASS: TestMigrateZeroTrustGatewayPolicy_V4ToV5_WithRuleSettings (18.12s)
=== RUN   TestMigrateZeroTrustGatewayPolicy_V4ToV5_WithNestedBlocks
--- PASS: TestMigrateZeroTrustGatewayPolicy_V4ToV5_WithNestedBlocks (18.78s)
=== RUN   TestMigrateZeroTrustGatewayPolicy_V4ToV5_ComplexSettings
--- PASS: TestMigrateZeroTrustGatewayPolicy_V4ToV5_ComplexSettings (19.80s)
=== RUN   TestMigrateZeroTrustGatewayPolicy_V4ToV5_EmptyRuleSettings
--- PASS: TestMigrateZeroTrustGatewayPolicy_V4ToV5_EmptyRuleSettings (18.84s)
PASS
ok  	github.com/cloudflare/terraform-provider-cloudflare/internal/services/zero_trust_gateway_policy	100.528s

Terraform Provider Migration test: #13