bump nim-ssz-serialization to `6f831b79df24af00c10e73e717cbe40d7d0e2439`

[etan-status]

• update ci.yml and fix deprecations
• Add two convenience aliases from specification

[github-actions[bot]]

Unit Test Results

         9 files  ±0    1 334 suites  ±0   31m 8s :stopwatch: - 1m 56s   5 000 tests ±0    4 652 :heavy_check_mark: ±0  348 :zzz: ±0  0 :x: ±0  20 880 runs  ±0  20 476 :heavy_check_mark: ±0  404 :zzz: ±0  0 :x: ±0 

Results for commit 0047f390. ± Comparison against base commit c0fc0f41.