Update code relying on hardcoded .monitoring template versions

[klacabane]

Parent #120825

In 8.x, Metricbeat will be writing its data in ECS compliant format. In order for stack monitoring to continue to support the legacy data format, new mappings have been created with the new ECS fields for indexing data, and alias fields for the legacy format which point to the corresponding ECS fields. See elastic/elasticsearch#81744

These new mappings will target a version 8 of the .monitoring-{product}-{version} indices and we need some updates to support that.

• update metricbeat.MakeXPackMonitoringIndexName to target version 8
  bump index name version beats#29493
• update stack monitoring, quick lookup shows these constants
• (optional) since the new indices won't be suffixed by -mb anymore, we should look into what this constant is used for and adapt the codepaths that relies on it.

[elasticmachine]

Pinging @elastic/infra-monitoring-ui (Team:Infra Monitoring UI)

[matschaffer]

https://github.com/elastic/kibana/blob/main/x-pack/plugins/monitoring/common/constants.ts#L126-L131 is no longer versioned. I think we can call this done.