Pin bundler version to `v2.4.22` on workflow runs

[howard-e]

Bundler has recently released v2.5+ which has dropped support for ruby 2.6 (and 2.7), which this repository's build process uses, so that has been a source of recent deployment failures.

The last supported version of bundler which can work with ruby 2.6.x is v2.4.22. This PR pins that dependency for the relevant workflow files (deploy.yml and build-only.yml).

Also verified this change with my own fork by running build-only.

[netlify[bot]]

‼️ Deploy request for wai-website rejected.

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🔨 Latest commit	c222463eba48af74e7869ca88392bcf0b2a5c264

[howard-e]

cc @shawna-slh