Migrate from Sprockets to Propshaft

[awilfox]

• We don't actually need a JavaScript runtime installed at all.
  • We don't transpile nor bundle anything.
  • dartsass handles the CSS compilation.
  • remove Yarn stuff from .gitignore since we don't use it any more.
• Update hardcoded asset paths to use helpers as appropriate.
• Remove unused code (empty application.js, Sprockets config, et c.)
• Use Ruby ~> 3.4 instead of hardcoding 3.4.9.
• Use SECRET_KEY_BASE_DUMMY instead of SECRET_KEY_BASE when precompiling assets in Docker.
• Update JSON Gem for vulnerability while we're here.
• Ensure Compose builds the Development target.

Implements: AP-808

[anarchivist]

one specific question about precompiling assets for dev, but other than that i'd say it looks good. inspecting the dartsass:build logs, it looks like we've got a bunch of deprecation warnings about using / as a division operator, but i suppose we could handle that separately.

[awilfox]

v2:

• remove yarn from .gitignore and a missed postCSS config file that is unused.
• update JSON gem for CVE
• use SECRET_KEY_BASE_DUMMY instead of SECRET_KEY_BASE in Docker.

[awilfox]

v3:

• Update CHANGES.md with changes.

[awilfox]

v4:

• change docker-compose.yml to use the development target like other apps. found while debugging the asset issue above, but doesn't change anything locally (both targets work correctly for me)