Write and publish Slack messages about upcoming changes in 0.19.0

[merelcht]

Description

Before the big 0.19.0 release and blog post, we should publish some messages on Slack highlighting several of the big changes coming and linking to migration guides where possible.

Context

We've got some big breaking changes coming in 0.19.0 and concerns have been raised in several meetings about whether users are prepared for what's coming. These messages on Slack are also a good opportunity to let users know we can assist them with migration if they need help.

[merelcht]

Changes to highlight:

1. Removal of ConfigLoader and TemplatedConfigLoader + migration: https://docs.kedro.org/en/stable/configuration/config_loader_migration.html
2. Removal of datasets from core Kedro package. Users should instead use kedro-datasets. + rename of DataSet to Dataset

We can already link to the release notes on develop and mention these are WIP: https://github.com/kedro-org/kedro/blob/develop/RELEASE.md

[merelcht]

Done!