Remove use of File.exists?

[silug]

Calls to File.exists? should be replaced with File.exist? for compatibility with Ruby 3.

[silug]

• https://github.com/simp/pupmod-simp-ima/blob/e44002e37a456f8d408820c2bc87a17bc23b1e2e/lib/facter/ima_log_size.rb#L4
• https://github.com/simp/pupmod-simp-ima/blob/e44002e37a456f8d408820c2bc87a17bc23b1e2e/lib/facter/ima_security_attr.rb#L25