Refactor - Makes loader-v4 the default

[Lichtso]

Problem

Loader-v4 was originally planned as a PRv2 loader, but will be used as default loader (superseding loader-v3) in PRv1 now.

Summary of Changes

• Renames and rekeys enable_program_runtime_v2_and_loader_v4 to enable_loader_v4.
• Disables new deployments on loader-v3 when the feature is enabled.
• WIP: Adjusts tests.

[mergify]

The Firedancer team maintains a line-for-line reimplementation of the
native programs, and until native programs are moved to BPF, those
implementations must exactly match their Agave counterparts.
If this PR represents a change to a native program implementation (not
tests), please include a reviewer from the Firedancer team. And please
keep refactors to a minimum.