Remove usage of Hedera in markdown files

[hendrikebbers]

Today "Hedera" is used in several Markdown files. You can get a search result here: https://github.com/search?q=repo%3Ahiero-ledger%2Fhiero-sdk-tck+Hedera+language%3AMarkdown&type=code&l=Markdown

We need to change it to Hiero or just removed.

Examples:

The hedera network must support at least 4 nodes -> The Hiero network must support at least 4 nodes

...can be set for all Hedera transaction types -> ...can be set for all transaction types

Next to the usage in Text the files contains a lot of links to the Hedera doc or Hedera repositories like protobuf. Those links should not be changed for now. We will fix it once all repos are transfered to Hiero.