Update scopes in Slack OAuth authorize URL

What does this MR do and why?

This merge request updates the scopes we request from Slack to match those we need to support Notifications &8670 (closed).

See #384226 (comment 1237783189).

Screenshots

integration_slack_app_notifications flag disabled	integration_slack_app_notifications flag enabled

How to set up and validate locally

To QA this feature you must have access to the Ecosystem Test Slack Workspace.

1. Start a new GitPod instance and configure the GitLab for Slack (dev) app to work with it (docs)
2. Checkout this feature branch, bundle and yarn, and restart GDK (docs):

    cd /workspace/gitlab &&
        git fetch origin 384226-add-additional-slack-scopes-to-oauth-authorize-url &&
        git checkout 384226-add-additional-slack-scopes-to-oauth-authorize-url &&
        bundle &&
        yarn &&
        cd /workspace/gitlab-development-kit &&
        gdk restart

3. Enable the flag. On the GitPod terminal: cd /workspace/gitlab && bundle exec rails c, then:

   Feature.enable(:integration_slack_app_notifications)

4. Choose a project, and set up the Slack application integration: Settings > Integrations. Choose GitLab for Slack app. Click Install Slack app
5. Notice the scopes that we are authorizing for described in the first step of the authorization flow from Slack.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #384226 (closed)