bump ruby version to 2.7.3

[no-reply]

2.7.3 has security fixes. let's test it.

@samvera/hyrax-code-reviewers

[no-reply]

in the past, we've done a hard swap over on circleci builds, but i think a more stepwise approach would suit better:

let's merge the new build as optional, give a couple of days to gauge stability and for rebases to happen, then change the GitHub config and remove the 2.7.2 build.

[no-reply]

looks like a genuine Ruby 2.7.3 support issue, to me.

[cjcolvar]

That's really surprising! I wouldn't have expected a ruby patch release to cause any test failures. I guess it shows the value of running tests on every change!

[stale]

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