batches: fix malformed links in deprecation banner

Created by: courier-new

Closes https://github.com/sourcegraph/sourcegraph/issues/21710.

This banner is likely going away in the next release anyway, but the links were malformed on managed instances and produced URLs like this:

https://sourcegraph.com/https://docs.sourcegraph.com/batch_changes/how-tos/bulk_operations_on_changesets

This just fixes that by switching them from Links to regular <a>s, since we want to keep the full, absolute link URL!