chore: Allow usage of hackney 4.x

[sax]

Description

Hackney has update several major versions in 2026, including security fixes. Looking at the hackney adapter, none of their breaking changes effect the sentry code.

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

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[sax]

Hmm the CI failure seems to be a setup issue with actions/create-github-app-token.

[sax]

Ok, looks like there is a transitive dependency conflict. :hackney 4.0.0 split out its HTTP/2 and HTTP3 stacks to an erlang_h2 library. Meanwhile, :opentelemetry_exporter depends on :grpcbox, which depends on :chatterbox... which implements a set of conflicting modules including :h2_connection. joedevivo/chatterbox#154

So technically the Sentry.HackneyClient is fine, but in practice one still can't update hackney to 4.0 until other libraries sort out the conflicts.