Refactor:Update delete source branchcheckbox in MR widget

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What does this MR do and why?

Refactor:Update delete source branchcheckbox in MR widget

• Add setRemoveSourceBranch function to MR widget store
• Change checkbox from v-model to @change event
• Add specs for new setRemoveSourceBranch function
• Update RSpec tests to reflect new checkbox behavior

Closes #387000 (closed)

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