Ensure deterministic order of frequency plans

[cvetkovski98]

Summary

Closes #6614.

Changes

• Store the gateway frequency plans under io.Connection in an array instead of a map to enforce consistent order

Testing

• Manual
• Configure multiple frequency plans for a gateway through the Console or by running ./ttn-lw-cli gtw set <GTW_EUI> --frequency-plan-ids US_902_928_FSB_1,US_902_928_FSB_2.
• Add a debug log entry for the sx1301_config sent to the gateway on connect and observe the logged value. I added this in lbslns.GetRouterConfig.
• Ensure the logged value is consistent between multiple reconnects.
• Running diff on the multiple outputs shows only a difference in the MuxTime.

Checklist

• Scope: The referenced issue is addressed, there are no unrelated changes.
• Compatibility: The changes are backwards compatible with existing API, storage, configuration and CLI, according to the compatibility commitments in README.md for the chosen target branch.
• Documentation: Relevant documentation is added or updated.
• The steps/process to test this feature are clearly explained including testing for regressions.
• Changelog: Significant features, behavior changes, deprecations and fixes are added to CHANGELOG.md.
• Commits: Commit messages follow guidelines in CONTRIBUTING.md, there are no fixup commits left.

[KrishnaIyer]

Looks good to me.