Update rust version to 1.98.0

[ozgb]

Bumps the pinned toolchain from 1.93.0 to 1.98.0 (latest stable) and VERSION from 0.18.4 to 0.18.5, following the same pattern as #99 and #95.

Why

We're using srtool to create deterministic wasm runtimes for the Midnight node - we've pulled in sysinfo@0.39.6 which needs rustc 1.95, so we're getting compile errors:

error: rustc 1.93.0 is not supported by the following package:
  sysinfo@0.39.6 requires rustc 1.95
Either upgrade rustc or select compatible dependency versions with
`cargo update <name>@<current-ver> --precise <compatible-ver>`
where `<compatible-ver>` is the latest version supporting rustc 1.93.0

1.95 would be enough to unblock us, but I've gone to 1.98.0 since only the latest stable release of Rust receives patches.

Please let me know if I can make any further updates here - thanks!