Support rails 8.1 version

Description

Loosen version requirement to allow usage in apps with rails 8.1.

Test fix

The Labkit::ExconPublisher spec needed a small update to land alongside this bump.

Rails 8.1 changed Object#to_param to return nil (instead of "") for nil values, so Hash#to_query now serializes a nil-valued key as the bare key (nil_key) rather than an empty assignment (nil_key=). See rails/rails#52610.

Labkit::ExconPublisher passes the Excon query hash through Hash#to_query when building the request.external_http ActiveSupport::Notifications payload, so the serialized :query field in that payload changes shape on Rails 8.1. The spec's hard-coded expectation ("abc=1&nil_key=&xyz=2") was updated to the new form ("abc=1&nil_key&xyz=2") to match.

Downstream impact

This is not a labkit-introduced behavior change — it's inherited from ActiveSupport and only manifests when the consuming app itself runs on Rails 8.1. Consumers pinned to Rails ≤ 8.0 see the old serialization unchanged. Consumers that parse the payload's :query string with any standard query parser get {nil_key => nil} either way; only consumers doing exact substring comparison on that field would notice.