[Heartbeat] Remove Deprecated certificate fields

[paulb-elastic]

The tls.certificate_not_valid_before and tls.certificate_not_valid_after fields have been deprecated since 7.8.0 (June 2020) in favour of tls.server.x509.not_before and tls.server.x509.not_after, so these can now be removed.

[elasticmachine]

Pinging @elastic/uptime (Team:Uptime)

[botelastic]

Hi!
We just realized that we haven't looked into this issue in a while. We're sorry!

We're labeling this issue as Stale to make it hit our filters and make sure we get back to it as soon as possible. In the meantime, it'd be extremely helpful if you could take a look at it as well and confirm its relevance. A simple comment with a nice emoji will be enough :+1.
Thank you for your contribution!

[botelastic]

Hi!
We just realized that we haven't looked into this issue in a while. We're sorry!

We're labeling this issue as Stale to make it hit our filters and make sure we get back to it as soon as possible. In the meantime, it'd be extremely helpful if you could take a look at it as well and confirm its relevance. A simple comment with a nice emoji will be enough :+1.
Thank you for your contribution!