[improve][build] Upgrade lightproto gradle plugin to 0.7.3

[merlimat]

Motivation

The lightproto Gradle plugin was upgraded to 0.7.2 in #25751. Version 0.7.3 picks up CVE fixes in the dependencies of the lightproto code generator.

Modifications

Bump the lightproto Gradle plugin version from 0.7.2 to 0.7.3 in gradle/libs.versions.toml.

Verifying this change

• Make sure that the change passes the CI checks.

This change is a trivial rework / code cleanup without any test coverage.

Verified locally that io.streamnative.lightproto:0.7.3 resolves from the plugin repository and that :pulsar-common:generateLightProto runs successfully.

Does this pull request potentially affect one of the following parts:

• Dependencies (add or upgrade a dependency)
• The public API
• The schema
• The default values of configurations
• The threading model
• The binary protocol
• The REST endpoints
• The admin CLI options
• The metrics
• Anything that affects deployment