Bump minimum ruby version requirement

[codlp]

WHY are these changes introduced?

Fixes #3816

Currently, the Shopify CLI's documentation indicates that, to work with themes, we need to have ruby version 2.7.2 or higher.

However, with ruby version 2.7.2 installed, running the command shopify theme pull --store {store-name} returns an error asking us to install ruby version 2.7.5 or higher.

WHAT is this pull request doing?

Bumping the minimum ruby version requirement in the documentation.

[isaacroldan]

@karreiro The docs are still not automated, so after merging this we need to open a PR to the docs. Also, our CI has a bug and doesn't currently work with remote branches, so you'd need to open a PR yourself to get this merged :/

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