[ruby/pathname] Officially drop support for ruby 2.6 or older

The gem doesn't even install on old rubies, but since the gemspec claims
it's supported, `gem install pathname` will try to install it and print
an error.

This commit doesn't fix the above issue. The only way to fix it would be
to restore support and release a new version that actually supports old
rubies. However, such a change has been proposed and ignored for a long
time.

So this issue proposes to leave that broken but at least bring the
gemspec manifest and the CI matrix in sync to hopefully avoid this issue
from happening again in the future.

https://github.com/ruby/pathname/commit/3ee010b538

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