resource/gitlab_project: Mark `topics` attribute as `Computed`

This avoids a subsequent non-empty plan to a minial gitlab_project configuration:

Note: Objects have changed outside of Terraform

Terraform detected the following changes made outside of Terraform since the last "terraform apply":

  # gitlab_project.test has changed
  ~ resource "gitlab_project" "test" {
        id                                               = "481"
        name                                             = "Test 3"
        tags                                             = []
      + topics                                           = []
        # (57 unchanged attributes hidden)

        # (2 unchanged blocks hidden)
    }

Unless you have made equivalent changes to your configuration, or ignored the relevant attributes using ignore_changes, the following plan may include actions to undo or respond to these changes.

This behavior has not yet been released. See #917