feat: Update default PAT prefix when not set

Updates the default Personal Access Token prefix to `glpat-` when not
previously set.

Relates to https://gitlab.com/gitlab-org/gitlab/-/issues/342327

Changelog: changed

db/migrate/20211112155416_populate_default_value_for_personal_access_tokens_prefix.rb

+# frozen_string_literal: true
+
+class PopulateDefaultValueForPersonalAccessTokensPrefix < Gitlab::Database::Migration[1.0]
+  def up
+    execute(
+      <<-SQL
+      UPDATE
+        application_settings
+      SET
+        personal_access_token_prefix = default
+      WHERE
+        personal_access_token_prefix IS NULL
+      SQL
+    )
+  end
+
+  def down
+    # no-op
+  end
+end

db/schema_migrations/20211112155416

+0c5627518093f6261679940402cbd756a91bf4617a37eecdbbc82dc57856dcec
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