feat!: remove Slack notification surface entirely

[venkatamutyala]

What

Removes the last remnants of Slack support — which was already deprecated and a no-op:

• enable_slack_notification_for_approval input definition (action.yml)
• the deprecation-notice step that emitted a warning when the input was set (action.yml)
• the input's row in the README inputs table

After this, there are zero slack references in the repo (verified with grep -rni slack).

Breaking change

Flagged as breaking (feat!: + BREAKING CHANGE: footer) because a documented input is removed from the action's public API → release-please will cut a major version (v6.0.0).

In practice the runtime impact is minimal: the input was already ignored, and GitHub silently ignores undeclared with: inputs, so existing workflows that still set enable_slack_notification_for_approval will continue to run (just without the deprecation warning). The major bump signals the API surface change to consumers.

Release

On merge, release-please will open/refresh a release PR proposing v6.0.0.

[github-actions]

⬇️ Download the full plan (tofu-plan.txt): https://github.com/GlueOps/github-actions-opentofu-continuous-delivery/actions/runs/28246478168/artifacts/7908891282

OpenTofu Plan

Show plan

OpenTofu used the selected providers to generate the following execution
plan. Resource actions are indicated with the following symbols:
  + create

OpenTofu will perform the following actions:

  # null_resource.integration_test will be created
  + resource "null_resource" "integration_test" {
      + id = (known after apply)
    }

Plan: 1 to add, 0 to change, 0 to destroy.

Changes to Outputs:
  + ok = "integration-test"