Fix warnings

[ankane]

Hi, this PR fixes warnings when Ruby warnings are enabled.

ruby -W -e 'require "vault"'

/home/user/.rbenv/versions/4.0.7/lib/ruby/gems/4.0.0/gems/vault-0.20.1/lib/vault/client.rb:4: warning: CGI library is removed from Ruby 4.0. Please use cgi/escape instead for CGI.escape and CGI.unescape features.
If you need to use the full features of CGI library, Please install cgi gem.
/home/user/.rbenv/versions/4.0.7/lib/ruby/gems/4.0.0/gems/vault-0.20.1/lib/vault/api/logical.rb:150: warning: assigned but unused variable - e
/home/user/.rbenv/versions/4.0.7/lib/ruby/gems/4.0.0/gems/vault-0.20.1/lib/vault/api/sys/mount.rb:127: warning: assigned but unused variable - json

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