test_bulk_insert: fix typing for PgVersion (#9854)

## Problem

Along with the migration to Python 3.11, I switched `C(str, Enum)` with
`C(StrEnum)`; one such example is the `PgVersion` enum.
It required more changes in `PgVersion` itself (before, it accepted both
`str` and `int`, and after it, it supports only `str`), which caused the
`test_bulk_insert` test to fail.

## Summary of changes
- `test_bulk_insert`: explicitly cast pg_version from `timeline_detail`
to str

test_runner/performance/test_bulk_insert.py

@@ -56,7 +56,7 @@ def test_bulk_insert(neon_with_baseline: PgCompare):
 
 def measure_recovery_time(env: NeonCompare):
     client = env.env.pageserver.http_client()
-    pg_version = PgVersion(client.timeline_detail(env.tenant, env.timeline)["pg_version"])
+    pg_version = PgVersion(str(client.timeline_detail(env.tenant, env.timeline)["pg_version"]))
 
     # Delete the Tenant in the pageserver: this will drop local and remote layers, such that
     # when we "create" the Tenant again, we will replay the WAL from the beginning.