stack snapshot.yml: prepare for breaking changes in 8.0.0

[mtojek]

This PR from @P1llus showed that we need to think about breaking changes between 7.x and 8.x, which result in different stack definitions.

The goal of this issue is to build a mechanism to prepare for differences in the stack snapshot.yml, so 7.15.0, 7.16.0 and 8.0.0 are supported.

[mtojek]

Let me evaluate the mess we have right now:

1. Kubernetes integration uses the 8.0.0 stack now (PR). It fails with Error: [config validation of [xpack.fleet].enabled]: definition for this key is missing
2. All integrations targetting 7.16.0 fail with [0m "stacktrace": ["org.elasticsearch.bootstrap.StartupException: java.lang.IllegalArgumentException: Context cache settings [script.context.ingest.cache_max_size, script.context.processor_conditional.cache_max_size, script.context.template.cache_max_size, script.context.template.max_compilations_rate] requires [script.max_compilations_rate] to be [use-context]",.

EDIT:

Issue no. 2 should be mitigated now, at least for integrations using Elasticsearch < 8.0.0.

[mtojek]

Both PRs merged, but we need to address these follow-ups:

• address FIXME introduced in Support different Kibana configs based on stack version #553
• refactor "profiles" logic, after some time we have spaghetti code there (switching to KibanaConfigFile affected too many places)

[mtojek]

I created these follow-ups, so we can close this issue.

#557
#558