Styling kbd tag in release posts consistent with docs

Why is this change being made?

The <kbd> tag is styled using the default Bootstrap values in release posts. The contrast of the background is distractingly high. This MR uses $color-gray-features and changes the styling to be more consistent with the GitLab docs styling.

Related to !100550 (merged) where the problem was discovered.

Direct link to Review App page with an example: https://release-post-styling-kbd.about.gitlab-review.app/releases/2018/11/22/gitlab-11-5-released/#epic-keyboard-shortcuts

Author Checklist

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• Added a description to this MR explaining the reasons for the proposed change, per say-why-not-just-what
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