tendermint: relax `Block` validation rules to avoid assumptions on initial height

[erwanor]

Fix #1435, by removing extra-validation done on the Block domain type.

• [x] Referenced an issue explaining the need for the change
• [x] Updated all relevant documentation in docs
• [x] Updated all code comments where relevant
• [ ] Wrote tests
• [x] Added entry in .changelog/

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 59.5%. Comparing base (1434375) to head (8e6d8ee). Report is 10 commits behind head on main.

:exclamation: Current head 8e6d8ee differs from pull request most recent head a3a6fee

Please upload reports for the commit a3a6fee to get more accurate results.

Files	Patch %	Lines
tendermint/src/block.rs	50.0%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1436 +/- ## ===================================== Coverage 59.5% 59.5% ===================================== Files 271 272 +1 Lines 26759 26846 +87 ===================================== + Hits 15927 15981 +54 - Misses 10832 10865 +33 ```

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