Failing to bundle on ubuntu-latest Ruby 2.5

[headius]

The ubuntu-latest Ruby 2.5 CI job has been failing for some time while trying to setup-ruby and bundle dependencies. I'm not sure when this started failing and I don't know what's causing it.

@eregon Has anything changed in setup-ruby recently that might cause this?

@tenderlove @kou @nobu @olleolleolle It seems to be an arity mismatch calling from RubyGems into safe_load, but the Ruby 2.5 job on Windows passes. Why would this behave differently on Linux?

[headius]

Example run: https://github.com/ruby/strscan/actions/runs/20728783709/job/59511107216

[kou]

It's caused by psych 5.3.0 released at 2025-11-10: https://github.com/ruby/psych/releases/tag/v5.3.0

We can't mix old RubyGems (2.7.6.3) and psych 5.3.0 or later (psych 5.2.6 is OK).

[kou]

RubyGems 2.7.6.3 and psych 5.2.6 worked: https://github.com/ruby/strscan/actions/runs/19098372905/job/54563988139

[headius]

We can't mix old RubyGems (2.7.6.3) and psych 5.3.0 or later (psych 5.2.6 is OK).

Should we start upgrading 2.5's RubyGems or downgrading the bundled psych? Or just drop 2.5 support?

[kou]

Hmm. Let's remove the Ubuntu + 2.5 CI job.

[kou]

#189