Investigate if 7.0 dashboards can work with 6.x indices

[simitt]

Following up on the PRs to align apm schema to ECS, mainly #1766 and #1783 we need to check if we can make the 7.0 dashboards work with 6.x indices by using aliases.

[simitt]

We could have a similar approach as the beats team has:
Make 7.0 dashboards work with 7.x indices, and 6.x dashboards work with 6.x indices, but ensure the dashboards can co-exist.

see elastic/beats#9998

[simitt]

Closing this as the dashboards and visualisations are no longer shipped with APM Server or Kibana. Instead they are now part of https://github.com/elastic/apm-contrib.