kvserver/liveness: liveness storage sends batch requests without admission header

**Describe the problem**

At least one source of the reports found in #112680 is likely the following to uses of the Get and Scan methods on`kv.DB` in liveness.Storage:

https://github.com/cockroachdb/cockroach/blob/a1cfb1b5573b1f606ea9527ae93025c2ef667e4e/pkg/kv/kvserver/liveness/storage.go#L73

https://github.com/cockroachdb/cockroach/blob/a1cfb1b5573b1f606ea9527ae93025c2ef667e4e/pkg/kv/kvserver/liveness/storage.go#L201

Replacing these two uses a db.Txn() avoids the warning in a good number of the reproductions I've seen locally. We likely don't want a Txn here though, so we'll want to use a different API that allows us to pass in an admission header.

Jira issue: CRDB-41842