[Accessibility]: Minor aria label cleanup

Created by: jasongornall

Audit type

ARC Toolkit

User journey audit issue

https://github.com/sourcegraph/sourcegraph/issues/34431

Problem description

Ark toolkit identified a couple missing aria elements. Should be able to lump these into 1 set of work. Contrast is a seperate ticket.

Expected behavior

Errors should be zero from the Arc toolkit on the diff view https://sourcegraph.com/github.com/sourcegraph/sourcegraph-code-discussions/-/commit/37873123e6fdf7706e6404f66d6a151e7d9a0cfa?visible=1#diff-b0678b51723ef8450026fa0f6967216fR22

Additional details

No response

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Owner

This issue will be fixed by GitStart. I have assigned the label 'gitstart' and added the assignee 'gitstart-sourcegraph'.