fix: update StartHeight signing info in AfterValidatorBonded hook when validator re-bonds

[julienrbrt]

Description

Closes: #10312

This PR fixes the mismatch between the behavior and the comment, as discovered in the issue above.

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Author Checklist

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• confirmed the correct type prefix in the PR title
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[codecov]

Codecov Report

Merging #11973 (cb9ba34) into main (23baecf) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main   #11973   +/-   ##
=======================================
  Coverage   65.97%   65.97%           
=======================================
  Files         677      677           
  Lines       71336    71341    +5     
=======================================
+ Hits        47061    47065    +4     
- Misses      21590    21591    +1     
  Partials     2685     2685           

Impacted Files	Coverage Δ
x/slashing/keeper/hooks.go	80.00% <100.00%> (+2.22%)	⬆️
crypto/keys/internal/ecdsa/privkey.go	82.45% <0.00%> (-1.76%)	⬇️

[alexanderbez]

LGTM