Match gl-button link hover color with gl-link

Let's update the hover link color of gl-button to be consistent with gl-link

Current	Proposal

Originally mentioned in this MR > gitlab!38733 (comment 392546789)

More information

@mvanremmerden: It seems like we are using $blue-500 for gl-button with the btn-link class (https://gitlab.com/gitlab-org/gitlab-ui/-/blob/master/src/components/base/button/button.scss#L396), whereas gl-link (as well as gl-deprecated-button) use $blue-600 (https://gitlab.com/gitlab-org/gitlab-ui/-/blob/master/src/components/base/link/link.scss#L3)