Update test values

[kevin-pease]

✨ What kind of change does this PR introduce? (Bug fix, feature, docs update...)

• Updates several test values which have changed after the recent Testnet reset;
• Removes several 'unused import' and 'unused variable' warnings;
• Fixes 2 code examples in the documentation.

Additionally, due to the new response data, it became clear that some values in some response models are optional. This was not specified in the Stellar documentation. The affected struct fields have been modified using Option<>.

🤔 Checklist before submitting

• All projects build
• Follows style guide lines
• Relevant documentation was updated
• Rebased onto current main