feat(uagents): set default network to mainnet

[jrriehl]

Proposed Changes

Set default network to mainnet and update logging messages accordingly.

All agents will still register by default on the Almanac API, but we switch the default network to mainnet. To register on testnet instead, agents should set the network to testnet explicitly with:

agent = Agent(..., network="testnet")

Types of changes

What type of change does this pull request make (put an x in the boxes that apply)?

• Bug fix (non-breaking change that fixes an issue).
• New feature added (non-breaking change that adds functionality).
• Breaking change (fix or feature that would cause existing functionality to stop working as expected).
• Documentation update.
• Something else (e.g., tests, scripts, example, deployment, infrastructure).

Checklist

Put an x in the boxes that apply:

• I have read the CONTRIBUTING guide
• Checks and tests pass locally

If applicable

• I have added tests that prove my fix is effective or that my feature works
• I have added/updated the documentation (executed the script in python/scripts/generate_api_docs.py)

[jrriehl]

Not used anywhere.