Add automation for updating main to the latest apm-data and go-docappender commits

[simitt]

Some core functionality for the apm-server moved to https://github.com/elastic/apm-data and to https://github.com/elastic/go-docappender some time ago. It is important to keep the apm-server updated with the latest features in bug fixes in these libraries. We should add automation for regularly updating the dependency in the main branch of the apm-server.

[inge4pres]

If we pull apm-data into apm-server via go mod, the simplest solution that comes to mind is to add a

go get github.com/elastic/apm-data@latest

as part of a git hook or a CI job on Pull Requests.
The hook or job should possibly commit the change to go.mod and go.sum if there was an update.

[kruskall]

Turned out dependabot does not support pseudo versions: dependabot/dependabot-core#2028

(thanks @axw for the link!)

Possible solutions are:

• semver releases for those libraries
• use updatecli similar to what we are doing for beats

[kruskall]

We've released stable versions, this should be fixed once the PR is merged