inputs: horizon-platform: dc6d92b1 -> 2e6a2ea4

inputs: horizon-platform: e187fb1a -> dc6d92b1

This is more reliable than trying to use the previous horizon-platform
version.

inputs: lint-utils: 5b97bd76dd6756dfc29a32d25a5c78c0522bac10 -> ee76287dc9987dd01ec38d38839b788a26ee5f24

apps.horizon-gen-nix: switch to 0.4.0

horizon-platform: 4b8c2685 -> e187fb1a

switch to horizon-gen-nix-0.2.1 with horizon-spec 0.2

Use dhall definitions from horizon-spec

horizon-gen-nix (flake input): f24f5cfb9c2629f9b456e47aedafdf8e909dadda -> 0b00a1c2c4e8f8ed169bb732ea35c4da4ea3e387