Use Pajamas button in _billing_plan_actions.html.haml

What does this MR do and why?

Use Pajamas button component for button in ee/app/views/shared/billings/_billing_plan_actions.html.haml

Issue: #416052

Screenshots or screen recordings

No visual changes to the "Upgrade" button.

Before	After

How to set up and validate locally

• Have a working CDot setup locally that's link with local GDK
• In GDK, navigate to a group's billing page
• Observe the "upgrade" button in the plans section

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.

marked the checklist item I have evaluated the MR acceptance checklist for this MR. as completed

added Leading Organization label

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assigned to @abhijeet007rocks8

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added Hackathon label

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@gitlab-bot ready

added workflowready for review label and removed workflowin dev label

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requested review from @brytannia

mentioned in issue gitlab-org/quality/triage-reports#13319 (closed)

added grouppersonal productivity maintenancerefactor labels

added typemaintenance label

added devopsmanage sectiondev labels

approved this merge request

Hey @abhijeet007rocks8 thanks for your contribution

backend LGTM

@nradina can you take frontend review, please? @alipniagov can you take maintainer backend , please?

requested review from @nradina and @alipniagov and removed review request for @brytannia

@stingrayza, thanks for approving this merge request.

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