Fix missing `Default (TxBuilderStrategy m)` constraint on `GYTxBuilderMonad`

[TotallyNotChase]

Summary

Very small change. I removed this constraint in a previous commit as part of #322 thinking it's redundant. But that's not correct since most use cases don't care about choosing a strategy when building transactions. So most use sites will just require a verbose Default (TxBuilderStrategy m) constraint on top. This improves upon that.

Type of Change

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• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [x] Code refactoring (improving code quality without changing its behavior)
• [ ] Documentation update (adding or updating documentation related to the project)

Checklist

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• [x] I have read the Contributing Guide
• [x] My code follows the project's coding style and best practices
• [x] My code is appropriately commented and includes relevant documentation
• [ ] I have added tests to cover my changes
• [x] All new and existing tests pass
• [x] I have updated the documentation, if necessary

Testing

Additional Information

[4TT1L4]

LGTM!

Thanks for the PR!