[ADD] Bootstrap 4 to 5 class renames

[bosd]

Odoo moved to Bootstrap 5 in 15.0, so the Bootstrap 4 direction classes stop doing anything from that version on. Nothing converts them, so they survive migrations. Counting files in OCA/web:

14.0: BS5=0 BS4=6
15.0: BS5=1 BS4=5
16.0: BS5=7 BS4=4

Four files on 16.0 still carry classes that have no effect.

This adds the 22 renames that have one unambiguous replacement: spacing (ml-, mr-, pl-, pr-), text alignment, float, border and rounded directions, dropdown alignment, the typography helpers, and sr-only, badge-pill, custom-select, no-gutters.

Left out: what Bootstrap 5 dropped without a replacement, like form-group, form-row, btn-block, media, jumbotron, card-deck, input-group-append and custom-control. Those need the markup rethought, so a warning suits them better than a rewrite. Can add that separately if you want it.

.xml only. Renaming a .ml-3 selector in a module's own scss would break that stylesheet instead of fixing it.

I put it in migrate_allways because the classes are inert from 15.0 on, so converting them is safe at any step, and a module that reached 17.0 with them still in place would never see a 14.0 to 15.0 script again. string_attribute_in_tree_view.yaml already works that way. Happy to move it to migrate_140_150, though there is no 14.0 fixture yet.

The renames come from the utilities API of the Bootstrap that Odoo bundles rather than from the changelog, so ms/me, ps/pe, text-start/text-end, float-start/float-end, fw- and fst-.

The fixture runs through the existing test_migration_180_190. It also covers what must not change: o_ml-2, my-ml-3, mt-2, mx-auto, text-lefty, o-text-right and ml-6.