Relax activesupport/hyperclient upper bounds; fix Faraday 2 Horizon client

[oartb]

(Supersedes #462, which closed itself when I renamed the source branch — same origin, no other changes there.)

Three fixes needed to use this gem under Rails 8 / Faraday 2, in the order I found them:

1. Relax activesupport upper bound. Both stellar-base and stellar-sdk cap
   activesupport < 8.0, which blocks any consumer from moving to Rails 8. The most
   recent release is 0.32.0 (2022-05-17) and main hasn't touched this constraint
   either. Widened to < 9.0.

2. Fix the Horizon client's Faraday 2 middleware reference. client.rb referenced
   FaradayMiddleware::FollowRedirects (the old, incompatible faraday_middleware
   gem's namespace) when the actual dependency, faraday-follow_redirects, registers
   Faraday::FollowRedirects::Middleware instead.

3. Relax hyperclient and require faraday-excon explicitly — this is the one that
   actually matters end-to-end. Fix is this dead ? #2 alone turned out to be necessary but not
   sufficient: hyperclient < 2.0 depends on the old faraday_middleware gem, which
   caps faraday < 2 transitively — so the dependency graph could never actually
   resolve to Faraday 2 regardless of what fix is this dead ? #2 changed, making it dead code in
   practice. hyperclient 2.0.0 drops faraday_middleware entirely and depends on
   faraday >= 2 + faraday-follow_redirects directly. Separately, faraday-excon
   needs an explicit require "faraday/excon" under Faraday 2 (same
   registration-by-require pattern as follow_redirects), and the specific release
   matters: 1.1.0's adapter class uses a dependency DSL macro that doesn't exist on
   Faraday 2's Adapter base class; 2.2.0+ does.

Verified end-to-end, not just resolved: with all three changes, bundle update hyperclient faraday_hal_middleware faraday-excon actually resolves faraday 2.14.3,
hyperclient 2.0.0, faraday-excon 2.2.0 (faraday_middleware disappears from the
graph entirely), and the full monorepo test suite passes under that real resolution:
base 364, sdk 53, horizon 33 examples, 0 failures. Without fix #3, everything
above still silently resolves to the old Faraday 1.x chain and never actually
exercises the Faraday-2 path fix #2 is for.

[codacy-production]

Not up to standards ⛔

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