Update observability to report on and use the build number log

[jloleysens]

We want to be able to know when older Kibana UI builds reach our Kibana server to help inform when it is OK to:

• generally know that old clients are still in use
• remove old, internal API versions

Note: this would be a step beyond just logging when an error happens as this is more of a passive observation that any old UI client has reached Kibana server.

[jloleysens]

Regarding removing old internal API versions, the Core team is proposing a different approach to start with:

When deleting an API version from Kibana code we should give it a sunset of 30d. This almost eliminates the possibility that an old client can still be in use because of the global session timeouts in place: users must refresh authentication tokens which requires a refresh of the Kibana client also.

[jloleysens]

It's likely the case that we want to log build numbers in our proxy logs if we want to address this comprehensively. Blocking this issue until we have sufficient justification to do so.