Rename helium to local testnet/regtest

[ldiego08]

Description

This PR renames helium to either regtest or local testnet, depending on the context. It provides a starting point to the migration from noble gas names to standard network names (regtest, mocknet, testnet, mainnet).

The following changes have been applied:

• Renamed the stacks-node helium subcommand to stacks-node regtest — The helium subcommand is intended to run Stacks with a local regtest Bitcoin node (for mocknet, there's a mocknet subcommand), hence it has been renamed to regtest.

• Refactored stacks-node code to use the term local testnet instead of helium — Beyond the CLI subcommand, "helium" is used interchangeably to refer to Stacks running with a local regtest or mocknet Bitcoin node (inferring it's for local development). Therefore, the helium.rs runloop file has been renamed to local.rs, comments now use "local testnet," and variables are updated to local_testnet.

If these names aren't the best, any suggestions are appreciated!

Based on a discussion with @wileyj

Applicable issues

• Relates to #4712 (comment)

Additional info (benefits, drawbacks, caveats)

Checklist

• [x] Test coverage for new or modified code paths
• [ ] Changelog is updated
• [x] Required documentation changes (e.g., docs/rpc/openapi.yaml and rpc-endpoints.md for v2 endpoints, event-dispatcher.md for new events)
• [ ] New clarity functions have corresponding PR in clarity-benchmarking repo
• [ ] New integration test(s) added to bitcoin-tests.yml