chore: bump CI to Node 24 and migrate ethers v6 usage

[3alpha]

Usage related changes

• Migrates L1/L2 postman test code to idiomatic ethers v6 usage.
• Replaces namespace import with named imports from ethers.
• Uses JsonRpcProvider directly.
• Uses getSigner() instead of listAccounts()[0].
• Updates ContractFactory and Contract references accordingly.
• Bumps ethers dependency range to ^6.16.0.

Development related changes

• Updates CI workflow Node runtime from 20 to 24 in build/test and publish jobs.
• Updates actions/checkout to v6.
• Updates actions/setup-node to v6.
• Updates softprops/action-gh-release to v3.
• Bumps @types/node to ^24.0.0 and refreshes lockfile accordingly.

Checklist:

• Applied formatting - npm run format
• No linter errors - npm run lint
• Performed code self-review
• Rebased to the latest commit of the target branch (or merged it into my branch)
• Updated the docs if needed
• Linked the issues resolvable by this PR
• Updated the tests if needed; all passing - npm test