fix: align discovery plugin package versions to 0.3.0

[mfaferek93]

Summary

bloom-release fails because 4 discovery plugin packages have version 0.1.0 while the rest of the repo is at 0.3.0:

ros2_medkit_beacon_common/package.xml:  0.1.0
ros2_medkit_linux_introspection/package.xml:  0.1.0
ros2_medkit_param_beacon/package.xml:  0.1.0
ros2_medkit_topic_beacon/package.xml:  0.1.0

bloom requires all packages in a repo to share the same version:

RuntimeError: Two packages have different version numbers (0.1.0 != 0.3.0)

Fix

Bump all 4 to 0.3.0 to match the rest of the repo. These packages were added in PRs #263 and #264 with 0.1.0 as initial version.

Blocks: first bloom-release to rosdistro (ros2-gbp/ros2_medkit-release created in ros2-gbp/ros2-gbp-github-org#977).