refactor: [index] Replace deprecated char with uint8_t in serialization

Summary:
All char representations are serialized in the same way, however the char one is deprecated. Also, using uint8_t directly avoids casts.

This is a backport of core#21824

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15933