Adds `twenty_upgrade_instance_info` — a new "info"-style gauge that
carries the inferred instance version (derived from the last applied
upgrade migration) as the `version` label.
This follows the standard Prometheus pattern for surfacing string-valued
metadata: value is always `1` (load-bearing for PromQL `group_left`
joins), the data lives on the label. Same shape as `node_uname_info`,
`go_info`, `kube_pod_info`, etc. — see [Prometheus naming
conventions](https://prometheus.io/docs/practices/naming/#metric-names).
On `/metrics`:
```
# HELP twenty_upgrade_instance_info Inferred instance version (semver-ish, derived from the last applied upgrade migration), carried as the `version` attribute
# TYPE twenty_upgrade_instance_info gauge
twenty_upgrade_instance_info{version="2.7.3"} 1
```
Also adds `MetricsService.createInfoGauge` as the helper for the pattern
(auto-suffixes `_info`, enforces value=1). Consumed by
twentyhq/twenty-eng#65.
