feat(sdk)!: change default network to mainnet

[shumkov]

Issue being fixed or feature implemented

If network is not specified, by default JS SDK connects to testnet. Since Platform is already deployed to mainnet we should change this.

What was done?

• Updated JS SDK to connect to mainnet by default.

How Has This Been Tested?

None

Breaking Changes

Now if network is not specified, JS SDK will connect to mainnet.

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added or updated relevant unit/integration/functional/e2e tests
• I have added "!" to the title and described breaking changes in the corresponding section if my code contains any
• I have made corresponding changes to the documentation if needed

For repository code-owners and collaborators only

• I have assigned this pull request to a milestone