Backport of 'Recreate SlackIntegration records on descendant integration on update'

What does this MR do and why?

Backports Recreate SlackIntegration records on descendant... (!202671 - merged), a severity2 bug fix, into 18.3. The original MR was included in 18.4 (chatops release check - 90 day retention policy)

Description from the original MR:

This MR updates Integrations::Propagation::BulkUpdateService to recreate SlackIntegration records when a Slack workspace has been installed on a parent integration. Previously, if a user removed and re-added a Slack workspace to a GitLab for Slack App integration with descendant integrations, the new Slack workspace wouldn't be installed for descendant integrations, leaving descendant integrations inactive.

MR acceptance checklist

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

• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
• The MR that fixed the bug on the default branch has been deployed to GitLab.com (not applicable for documentation or spec changes).
• The MR title is descriptive (e.g. "Backport of 'title of default branch MR'"). This is important, since the title will be copied to the patch blog post.
• Required labels have been applied to this merge request
  • severity label and bug subtype labels (if applicable)
  • If this MR fixes a bug that affects customers, the customer label has been applied.
• Set the milestone of the merge request to match the target backport branch version.
• This MR has been approved by a maintainer (only one approval is required).
• Ensure the e2e:test-on-omnibus-ee job has either succeeded or been approved by a Software Engineer in Test.

Note to the merge request author and maintainer

If you have questions about the patch release process, please:

• Refer to the patch release runbook for engineers and maintainers for guidance.
• Ask questions on the #releases Slack channel (internal only).
• Once the backport has been merged, the commit changes will be automatically deployed to a release environment that can be used for manual validation. See after merging runbook for details.