minor #15470 Upgrade popper package to v2 (ucay)

This PR was submitted for the 5.3 branch but it was merged into the 4.4 branch instead. Discussion ---------- Upgrade popper package to v2 Bootstrap 5 is using popper v2 Based on https://popper.js.org/docs/v2/migration-guide/ from `popper.js` to ``@popperjs`/core` <!-- If your pull request fixes a BUG, use the oldest maintained branch that contains the bug (see https://symfony.com/releases for the list of maintained branches). If your pull request documents a NEW FEATURE, use the same Symfony branch where the feature was introduced (and `5.x` for features of unreleased versions). --> Commits ------- 608de4a Upgrade popper package to v2

Author: javiereguiluz

frontend/encore/bootstrap.rst

@@ -43,7 +43,7 @@ used in your application:
 .. code-block:: terminal
 
  // jQuery is only required in versions prior to Bootstrap 5
- $ yarn add jquery popper.js --dev
+ $ yarn add jquery @popperjs/core --dev
 
 Now, require bootstrap from any of your JavaScript files: