replace FORCE_JAVASCRIPT_ACTIONS_TO_NODE24 with real action upgrades

[parsakhaz]

all workflows are opted into node 24 via the FORCE_JAVASCRIPT_ACTIONS_TO_NODE24 env flag to get ahead of github flipping the default on june 16, 2026. works, but it's a stopgap.

proper fix:

• bump actions/checkout, actions/setup-node, actions/cache, actions/upload-artifact, actions/setup-python and pnpm/action-setup to majors that target node 24 natively
• check peter-evans/repository-dispatch (notify-website.yml) for a node 24 release
• drop the env flag from all workflows once everything's bumped

node 20 gets removed from runners sept 16, 2026, so this needs to land well before then.