[target allocator] Switch to Prometheus 3.0 defaults

[swiatekm]

Prometheus 3.0 introduces a new scrape protocol: PrometheusText0.0.4 and it adds it to default scrape protocols. Prometheus instances <3.0 and prometheus receivers <0.120.0 don't know about this protocol, so they don't accept this configuration. This means that upgrading to Prometheus 3.0 libraries makes target allocator emit configurations incompatible with these versions. #3831 overrides these defaults to preserve compatibility.

We do want to switch to Prometheus 3.0 defaults sooner rather than later. This issue exists to track this effort. The plan is to switch in operator 0.124.0.