Applies_to: Forbid versions for non-versioned products

[florent-leborgne]

Serverless (and children of serverless) should accept only a lifecycle and not a version:

• serverless: ga is ok.
• serverless: ga 9.5 is not ok.

Elastic Cloud Hosted (ess) should accept only a lifecycle and not a version:

• ech: preview is ok.
• ech: preview 9.5 is not ok.

When this is incorrectly specified, the build should fail. A possible hint for this scenario is:
Can't specify a version for $key because this product is not versioned. If there is a specific minimum stack version to specify, indicate it as requirement or reconsider the keys used in the applies_to metadata.

cc @shainaraskas following the docs-eng sync