BSV: Replace `mr-2` in `bv-toast.js`

This issue concerns a file within the vendored BootstrapVue directory. At the time of this issue’s creation, the file exists and uses the CSS class named in this issue's title.

The general plan is to replace BootstrapCSS utility CSS classes with Tailwind equivalents, as outlined in the parent epic. However, in certain cases (see BFormRadio/BFormCheckbox, here and here, and BButton here and here), we have determined that we may not want to support the feature associated with this CSS class.

Tasks for this issue:

1. Investigate the purpose of this CSS class.
2. If the feature is documented in gitlab-ui and we want to keep supporting it:
   1. Replace the Bootstrap class with a Tailwind equivalent. Done.
3. If the feature is not documented in gitlab-ui and we decide not to support it:
   1. Check whether the GitLab main repository or any satellite projects currently use this feature, triggering this CSS class.
      1. If no: Remove the code related to the feature using this CSS class.
      2. If yes: Create an integration branch with an MR for gitlab-ui to update the project.

The effort for this issue should be time-boxed. If removing the feature and potential traces is too time-consuming, consider replacing the Bootstrap CSS utility class with an equivalent as an interim solution, as the primary goal is to unblock the overall objective of removing Bootstrap utility styles in gitlab-ui. In this case, findings from this issue should be added to the specific task for migrating this component from BootstrapVue later: &15178