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.
• Adds loader-v4 to write lock demotion exemptions. Otherwise loader-v4 programs could not be modified as they would always be readonly.
• WIP: Adjusts tests:
• Removes test_program_upgradeable_locks() (which was introduced in Prevent the invoke and upgrade of programs in the same tx batch solana-labs/solana#14653) and is superseded by test_program_sbf_invoke_in_same_tx_as_redeployment().
• Merges test_program_sbf_invoke_upgradeable() into test_program_sbf_upgrade().
• Merges test_program_sbf_invoke_upgradeable_via_cpi() and test_program_sbf_set_upgrade_authority_via_cpi() into test_program_sbf_upgrade_via_cpi().

[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.