New `align-self: center` default for btn class causes regression in vertical button stacks inside flex containers

Created by: andreas-venturini

Consider the following snippet (slightly modified example from https://github.com/twbs/bootstrap/pull/34834#issue-981860825)

<div class="d-flex flex-column">
  <button type="button" class="m-2 btn btn-primary">Default</button>
  <button type="button" class="m-2 btn btn-primary btn-sm">Small</button>
  <button type="button" class="m-2 btn btn-primary btn-md">Medium</button>
  <button type="button" class="m-2 btn btn-primary btn-lg">Large</button>
</div>

Bootstrap 5.1.1 produces the following output:

With Bootstrap 5.1.2 and align-self: center this results in the following: