feat(destinations): validate no update on slack

[nzur-newrelic]

Description

The current documentation shows to use ignore_changes on slack destinations, but sadly this situation causes issues with our mutations, therefore update for it will never work. To avoid this issue we'll allow users only to destroy the resource if needed.

A validation has been added for slack to avoid updating it as well in case someone didn't see the docs.

Type of change

Please delete options that are not relevant.

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [x] This change requires a documentation update

Checklist:

Please delete options that are not relevant.

• [ ] My commit message follows conventional commits
• [ ] My code is formatted to Go standards
• [ ] I have performed a self-review of my own code
• [ ] I have made corresponding changes to the documentation
• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] New and existing unit tests pass locally with my changes. Go here for instructions on running tests locally.

How to test this change?

Please describe how to test your changes. Include any relevant steps in the UI, HCL file(s), commands, etc

• Create a slack destination (through the UI)
• Import it to terraform
• Try to update the destination