chore(runtime): bump spec_version to 8

[nol4lej]

Ships the audit-remediation batch and the preceding config/feature work as one forkless upgrade. transaction_version stays at 2 — no call index or dispatch signature changed; only the metadata moves, which spec_version tracks.

Included since spec 7:

• validator-set 0.2.0: registration bond removed (Refactor/runtime config modules #121)
• shielded-pool: minimum shield amount removed (Refactor/runtime config modules #121)
• shielded-pool: sealed-tree Merkle-node pruning (Feat/prune sealed merkle nodes #122)
• runtime config split into modules (Refactor/runtime config modules #121); legacy shieldedPool_* RPC retired (fix(rpc): retire the legacy shieldedPool_* server #119)
• zk-verifier: deserialization bounds + circuit-id de-aliasing (fix(zk-verifier): bound deserialization input and stop circuit-id aliasing #123)
• shielded-pool: bounded zero-hash ladder and tree-depth shift (fix(shielded-pool): bound the zero-hash ladder and the tree depth shift #124)
• shielded-pool 0.16.0: non-canonical / zero commitments and nullifiers refused (fix(shielded-pool)!: reject non-canonical and zero commitments and nullifiers #125)
• precompile 0.5.0: ABI decoder truncation/overflow fixed (fix(precompile): reject truncated and overflowing ABI words in the shielded-pool decoder #126)
• shielded-pool 0.17.0: duplicate nullifier in private_transfer refused (fix(shielded-pool): reject two equal non-dummy nullifiers in private_transfer #127)

RUNTIME_VERSIONS.md reformatted to a categorized changelog for this release.