Make compatible with 'proper' custom_elements:3.x releases

Problem/Motivation

Custom Elements module 3.x has progressed to 'proper alpha status'. (3.0.0-alpha1 released in May, alpha2 last week.)

This module composer.json (since November 2022 / v2.3.0) allows "drupal/custom_elements": "^2.0 || 3.x-dev". In practice (as observed) this means that if any dependent project requires 3.0.0-alpha2, composer will actually include the latest 3.x-dev version, because this module does not allow to include 'proper' 3.x releases yet.

Steps to reproduce

Have a project that includes this module and requires "drupal/custom_elements": "^3.0.0-alpha2"

Run composer update --prefer-lowest --with-dependencies; see the latest commit from the 3.x branch (newer than alpha2) being installed.

Proposed resolution

Change requirement to "drupal/custom_elements": "^2.0 || ^3.0.0-alpha2".

Compatibility break

I expect that, once this change is done in this and lupus_decoupled modules, projects will 'revert' to not being able to install -dev versions by default / will not be able to update to the latest lupus_ce_renderer version unless they explicitly downgrade to a non-dev version of custom_elements.

AFAICT introducing that 'incompatibility' is a good thing:

• AFAICT most modules don't explicitly allow -dev versions in their composer.json, probably for good reason / because it makes things more predictable.
• custom_elements 3.x-dev is currently extremely close to alpha2 version, so now is the time to make this 'break'.
• usage statistics at this moment still show 0 installs for the custom_elements 3.x version.