resource/cloudflare_custom_ssl: handle when updates remote ID

[jacobbednarz]

Supersedes/closes #1025
Closes #1806

[jacobbednarz]

acceptance tests are passing

TF_ACC=1 go test $(go list ./...) -v -run "^TestAccCloudflareCustomSSL_" -count 1 -parallel 1 -timeout 120m -parallel 1
?       github.com/cloudflare/terraform-provider-cloudflare     [no test files]
=== RUN   TestAccCloudflareCustomSSL_Basic
--- PASS: TestAccCloudflareCustomSSL_Basic (10.12s)
=== RUN   TestAccCloudflareCustomSSL_WithEmptyGeoRestrictions
--- PASS: TestAccCloudflareCustomSSL_WithEmptyGeoRestrictions (11.67s)
PASS
ok      github.com/cloudflare/terraform-provider-cloudflare/internal/provider   22.353s
?       github.com/cloudflare/terraform-provider-cloudflare/tools/cmd/changelog-check   [no test files]
?       github.com/cloudflare/terraform-provider-cloudflare/tools/cmd/maintainer-only-file-check        [no test files]
?       github.com/cloudflare/terraform-provider-cloudflare/tools/cmd/tf-log-check      [no test files]
?       github.com/cloudflare/terraform-provider-cloudflare/version     [no test files]

[github-actions]

changelog detected ✅

[github-actions]

This functionality has been released in v3.21.0 of the Terraform Cloudflare Provider.

Please see the Terraform documentation on provider versioning or reach out if you need any assistance upgrading.

For further feature requests or bug reports with this functionality, please create a new GitHub issue following the template. Thank you!