Update ruby matrix: remove 2.6, add 3.1

[CAMOBAP]

Intro

Original task: https://github.com/metanorma/metanorma-build-scripts/issues/79

Problems:

• https://github.com/metanorma/metanorma/runs/5526293045?check_suite_focus=true#step:3:399-419
• https://github.com/metanorma/metanorma-docker/runs/5526676957?check_suite_focus=true

Blocks:

• https://github.com/metanorma/metanorma-docker/issues/133
• https://github.com/metanorma/metanorma-docker-private/issues/1

So we need to have a prebuild-gem for 3.1 and release it ASAP

[CAMOBAP]

A single failure is a known issue https://github.com/lutaml/expressir/issues/74. Previously it was observed Mac, so I propose to address it separately

[ronaldtse]

Agree with @CAMOBAP . Merging now.