Update capitalization of beta and experiment in ui

What does this MR do and why?

As part of the related MR, we're standardizing on lowercase for "beta" and "experiment", so we should update the UI text to match.

Additionally, Pajamas now has updated language with this badge's on-hover, but it seems we never got around to updating it in the code to match, so this MR also aligns the language with: https://design.gitlab.com/usability/feature-management#highlighting-feature-versions

• Related to Updating case of beta and experiment (!153188 - merged)
• Related to Docs: Update experiment, beta, ga to match lowe... (#461651 - closed)
• Related to Update the UI text for the experimental/beta fe... (gitlab-org/gitlab-services/design.gitlab.com#1763 - closed)
• Related to Update UI text in the feature management section (gitlab-org/gitlab-services/design.gitlab.com!3710 - merged)
• Related to feat(content): Lowercase for beta and experiment (gitlab-org/gitlab-services/design.gitlab.com!3853 - merged)

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