Shift required_ruby_version to v3.1.4

[huwd]

What’s changed

Minimum supported ruby is now 3.1.4

Breaking change: 2.7x was end of support 31 Mar 2023 We'll run into dependency issues (expectations having moved up) if we don't upgrade.

Let's move the testing to target the 3x series, but enforce 3 as a new minimum.

Identifying a user need

We need to keep our dependencies up to date, or this repo should be archived