Fix errors and panics in the chirpstack v3 source

[KrishnaIyer]

Summary

Fix errors and panics in the chirpstack v3 source. Ref: https://github.com/TheThingsIndustries/lorawan-stack-support/issues/1029

Changes

• Pull Custom CA file only if CA path is set and insecure is not set.
• Replace the deprecated grpc.WithInsecure() method.
• Remove the ClientConn field on the Source so that the underlying one from the Config may be used.

Testing

Local with a ChirpStack v4 instance. This of course doesn't check actual migration but makes sure that the tool doesn't panic.

This will be tested by CE as a part of https://github.com/TheThingsIndustries/lorawan-stack-support/issues/1029.

Regressions

NA. This is fixing existing bugs.

Checklist

• Scope: The referenced issue is addressed, there are no unrelated changes.
• Documentation: Relevant documentation is added or updated.
• Changelog: Significant features, behavior changes, deprecations and fixes are added to CHANGELOG.md.