op-challenger: fixes latestBlockNumber signature and ROOT_CLAIM length

[joseandro]

Description
Two issues were found while running the op-challenger && make alphabet command with foundry 0.2.0. Those issues prevent users from playing Alphabet's bisection game:

1. The public view parameters in latestBlockNumber() found in op-challenger/scripts/create_game.sh were causing cast call to fail.
2. The slicing of ROOT_CLAIM in op-challenger/scripts/alphabet/init_game.sh was yielding less than 32 bytes.

Tests
Executed the existing scripts in op-challenger && make alphabet successfully using foundry's toolset version 0.2.0.

[codecov]

Codecov Report

Merging #8136 (cdece16) into develop (079a99b) will decrease coverage by 0.51%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop    #8136      +/-   ##
===========================================
- Coverage    53.83%   53.33%   -0.51%     
===========================================
  Files          162      162              
  Lines         6171     6171              
  Branches       966      966              
===========================================
- Hits          3322     3291      -31     
- Misses        2723     2756      +33     
+ Partials       126      124       -2     

Flag	Coverage Δ
cannon-go-tests	63.47% <ø> (ø)
chain-mon-tests	26.95% <ø> (ø)
common-ts-tests	26.74% <ø> (ø)
contracts-bedrock-tests	60.31% <ø> (-1.58%)	⬇️
contracts-ts-tests	100.00% <ø> (ø)
core-utils-tests	44.03% <ø> (ø)
sdk-next-tests	41.92% <ø> (ø)
sdk-tests	41.92% <ø> (ø)

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see 7 files with indirect coverage changes

[ajsutton]

LGTM. Thanks for updating this.