alias `ech` for Elastic Cloud Hosted in `applies_to`

[shainaraskas]

Discussed in #549

^{Originally posted by shainaraskas February 19, 2025}
ess (Elasticsearch Service) is an outdated term, and most people should get used to not using it anymore. let's allow for ech to be added to applies_to, as an alternative to ess

I'm also ok with just renaming ess to ech but it will require cleanup we might not be able to get to quickly

[bmorelli25]

Related tickets to work on at the same time:

• invalid applies_to keys can be set in front matter #610
• empty applies_to tag strangeness #609