Remove taintedness as it was removed in Ruby 3.2

[jkorpela]

Merging inifiles does not work because in dup it attempts to taint which is no longer available in 3.2.

[smortex]

For reference, these function have been no-op since Ruby 2.7 and where removed in Ruby 3.2: https://bugs.ruby-lang.org/issues/16131

@TwP, do you still have enough cycles to handle this gem and do release?

[graaff]

There are also test for tainted? that should be removed from test_inifile.rb

[joshcooper]

@jkorpela couid you update your PR to remove tainted? from the tests?