Remove "btn" class from buttons.scss

What does this MR do and why?

Before we had styling button stylings coming from "gl-button", they came from "btn" from bootstrap and then we overrode the stylings in buttons.scss. In our current state, all buttons coming from Pajamas also have a class of "btn" and the bootstrap styles are overwritten in buttons.scss in GitLab-UI (https://gitlab.com/gitlab-org/gitlab-ui/-/blob/main/src/components/base/button/button.scss)

So this class was remaining to style any existing components with the class of "btn" but no class of "gl-button", in these instances I've added the renamed "btn" class name of "quarantined-deprecated-btn" so that it will still receive the same css at the same specificity.

Please note: I didn't update any buttons in the vendored "sidekiq-7.2.4"

There are 15 instances where we use styling with the class of "btn" but no class of "gl-button". An epic was created to detail the migrations to be done, implementation guide added and Seeking community contributions and quick win labels are added to each of the issues: &15411

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Related to #497234