Fix Ruby 2.7 warnings for git_instance_proxy_spec.rb

What does this MR do?

Issue #257438 (closed)

In Ruby 3.0, positional arguments and keyword arguments will be separated. Ruby 2.7 will warn for behaviors that will change in Ruby 3.0. In most cases, you can avoid the incompatibility by adding the double splat operator. It explicitly specifies passing keyword arguments instead of a Hash object.

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Conformity

• [-] Changelog entry
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• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
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