Remove social_pwa:activity_send_push dependency from social_pwa

Problem/Motivation

Social PWA has a dependency on it's submodule, this causes Drupal's Packagist to add a dependency of drupal/activity_send_push: * to the composer.json and drupal/activity_send_push because it's a submodule has a dependency on a specific version of drupal/social_pwa causing a "circular reference".

This reference create an issue where we can't update social_pwa:^2.0.0 with the followgin issue:

Problem 1
    - Root composer.json requires drupal/social_pwa ^2.0, found drupal/social_pwa[dev-2.0.x, 2.0.0, 2.0.x-dev] but these were not loaded, likely because it conflicts with another require.

For those on drupal slack, we started a discussion about how to better solve this: https://drupal.slack.com/archives/C51GNJG91/p1696929217161989

Steps to reproduce

With an already installed version of social_pwa, lower than 2.0.0, do a composer require/update social_pwa:^2.0.0

Proposed resolution

Remove social_pwa:activity_send_push from social_pwa.info

Because we still want to be able to have everything working by just enabling social_pwa we add an install hook to enable the module post-installation, but this install hook doesn't affect the generated composer.json metadata.

Remaining tasks

User interface changes

API changes

Data model changes