Use SHA256 instead of SHA1 where needed in tests.

[jackorp]

Systems such as RHEL 9 are moving away from SHA1 disabling it completely in default configuration.

This is a follow-up PR after https://github.com/ruby/openssl/pull/507. This PR has the goal of fixing the rest of the test failures [0] present on systems that disable SHA1, like RHEL 9 and Centos 9 stream, by default via a system-wide crypto policy.

Tests pass locally with these changes.

[0] https://gist.github.com/jackorp/bb6e3ff2cfd339f2f65afcdc5c9e4070

This supersedes https://github.com/ruby/openssl/pull/511

[jackorp]

I decided to redo the change completely instead of trying to fix I had done in #511 .

[rhenium]

Thank you!