Align expected variables and variables used in templates

Problem/Motivation

There are some differences between expected variables ("attributes", slots, props, declared in templates, macro arguments...) and callables (provided by Twig, defined as macro) and variables used in templates (printed, tested, altered...)

We need to align them.

Proposed resolution

Bootstrap 5:

• table: table_classes is used in template but not defined

Let's do a MR.

Bootstrap 4:

• alert: alert is used in templates but never defined >> it must be a string instead
• blockquote: variants are defined but not managed in templates >> remove them
• dropdown: split is used in template but never defined >>a leftover, let's remove it
• figure: no variant in definition, but variant management in template
• table: table_classes is used in template but not defined

Let's commit directly.

API changes

Nothing, because we are lucky: we may change only templates and not the definitions.