Change namespace from Hedera/Hashgraph to Hiero

[hendrikebbers]

• Usage of "Hedera": https://github.com/search?q=repo%3Ahiero-ledger%2Fhiero-sdk-js%20Hedera&type=code
• Usage of "Hashgraph": https://github.com/search?q=repo%3Ahiero-ledger%2Fhiero-sdk-js+Hashgraph&type=code

We need to refactor most of that to use "Hiero" / "hiero.org" / ...
Here a simple "replace all" is not the right solution since "Hedera Mainnet" will still be "Hedera Mainnet" while "a Hedera network" must be renamed to "a Hiero based network".

Package deployment will happen under "https://www.npmjs.com/org/hiero-ledger" in future.

I'm happy to review such a PR and give advice for it.