Adjust runner instructions modal in small screens

What does this MR do?

Adjust runner instructions modal in small screens

This change captures resizes of the runner installation instructions modal to make it usable on xs screens.

Screenshots (strongly suggested)

Changes only impact xs (< 576 px) viewports.

before	after

Does this MR meet the acceptance criteria?

Conformity

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Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
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Security

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• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
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Related to #327124 (closed)