Update sizes used in the loading icon (spinner) component

What does this MR do and why?

Partly solves #29847 (closed)

Updating spinner component to use size lg where md is currently used.

*There are currently ~77 instances of size="md" - it will be cleaner/easier to take this in steps. This is the last MR for this replace.

Screenshots or screen recordings

number	md	lg
1
2
3
4
5

How to set up and validate locally

number	file	steps
1	/vue_merge_request_widget/components/mr_widget_status_icon.vue	Navigate to - < any merge request >
2	/vue_merge_request_widget/components/mr_widget_pipeline.vue	Navigate to - < any merge request >
3	/sidebar/components/time_tracking/report.vue	Navigate to - < any issue > > comment /spend 1h > click on timetracking report in the sidebar
4	/repository/components/preview/index.vue	Navigate to - < any repository >
5	/repository/components/last_commit.vue	Navigate to - < any repository >

Confirmed/validated manually, example in screenshots.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

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