Fix a NoMethodError on Ruby 2.7 caused by Hash#except

## Motivation and Context

The gemspec already declares `required_ruby_version >= 2.7.0`, but the conformance Rake task used `Hash#except` to drop
the `:port` option before handing the remaining options to `Conformance::ClientRunner`. `Hash#except` was only added in Ruby 3.0,
so running `rake conformance` on Ruby 2.7 aborted before any scenario ran, raising `NoMethodError: undefined method except for {}:Hash`.

CI missed this because no job runs `rake conformance` on Ruby 2.7: the conformance workflow runs only on Ruby 4.0,
while the Ruby 2.7.0 matrix entry runs `rake test` alone.

## How Has This Been Tested?

Replace `options.except(:port)` with `options.reject { |key, _| key == :port }`, which has been available since well
before Ruby 2.7 and returns an equivalent Hash without mutating the original. `rake conformance` now runs the full suite on Ruby 2.7,
and its baseline still passes on current Ruby, verified on Ruby 4.0.

## Breaking Changes

None.

Rakefile

@@ -30,7 +30,7 @@ task :conformance do |t|
   options[:verbose] = true if ENV["VERBOSE"]
 
   Conformance::ServerRunner.new(**options).run
-  Conformance::ClientRunner.new(**options.except(:port)).run
+  Conformance::ClientRunner.new(**options.reject { |key, _value| key == :port }).run
 end
 
 desc "List available conformance scenarios"