refactor: Uses the new create-only plan modifier in flex cluster#3658

EspenAlbert wants to merge 4 commits intomastermongodb/terraform-provider-mongodbatlas:masterfrom

CLOUDP-343190_flex_delete_on_create_timeoutmongodb/terraform-provider-mongodbatlas:CLOUDP-343190_flex_delete_on_create_timeoutCopy head branch name to clipboard