Enable 8.1.0-SNAPSHOT for CI again

[devcorpio]

Context

The synthetics E2E pipeline was failing when executing tests for 8.1.0-SNAPSHOT due to an unhealthy container: ERROR: for elastic-agent Container "dada48cb7ab1" is unhealthy.

The reason of this issue was visible in the logs of that container:

Kibana Fleet setup failed: http POST request to http://kibana:5601/api/fleet/setup fails: Unauthorized: <nil>. Response: {"statusCode":401,"error":"Unauthorized","message":"Unauthorized"}

Issue where the agent team is working on a fix: elastic/beats#29603

What to do

Since we disabled 8.1.0 to avoid having the pipeline failing repeatedly. What we need to do is enable it again once the fix is ready.

[awahab07]

Post FF Testing

LGTM: The pipeline is consistently stable over the past CI runs.

Last successful run of the time of writing https://apm-ci.elastic.co/job/apm-agent-rum/job/e2e-synthetics-mbp/job/master/80/