Revert required encryption on CI runner tokens

This reverts https://gitlab.com/gitlab-org/gitlab/-/merge_requests/31800
because there may be runners that have not had their tokens encrypted,
and that merge request breaks those runners.

We should issue another migration/check to ensure that
`Ci::Runner#ensure_token` has been run before we make this required.

Relates to https://gitlab.com/gitlab-org/gitlab/-/issues/245322

app/models/ci/runner.rb

@@ -10,7 +10,7 @@ module Ci
     include TokenAuthenticatable
     include IgnorableColumns
 
-    add_authentication_token_field :token, encrypted: -> { Feature.enabled?(:ci_runners_tokens_optional_encryption) ? :optional : :required }
+    add_authentication_token_field :token, encrypted: -> { Feature.enabled?(:ci_runners_tokens_optional_encryption, default_enabled: true) ? :optional : :required }
 
     enum access_level: {
       not_protected: 0,

changelogs/unreleased/revert-a6875e5f.yml

+---
+title: Revert required encryption on CI runner tokens
+merge_request: 42623
+author:
+type: fixed

config/feature_flags/development/ci_runners_tokens_optional_encryption.yml

 ---
 name: ci_runners_tokens_optional_encryption
-introduced_by_url: 
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/commit/cbba8fe02b0ea3adabcfe18685c9af6be3859e2d
 rollout_issue_url: 
-group: 
+group: group::continuous integration
 type: development
-default_enabled: false
+default_enabled: true