[APM] Fleet: Show warning if apm package and Kibana is not the same version

[sorenlouv]

If a user upgrades Kibana from 7.15 to 7.16 the APM integration package is not automatically updated accordingly (this is expected to happen by 8.0).

Thus, for 7.16 a warning should be displayed to users, urging them to upgrade their APM package, if it is not aligned with their Kibana version.

[elasticmachine]

Pinging @elastic/apm-ui (Team:apm)

[simitt]

Ignore the patch version, checking for 7.16 should be enough.

[ogupte]

Along with a warning message, the migration UX should probably be disabled:

If there's a mismatch between the stack version and package version, extra input vars would be assigned to the policy which might fail policy creation. Even if it doesn't fail, the user will be under the impression that some of their configurations would be migrated, but since it's an older apm package, those configs will not reach apm-server.