Resolve deprecation warnings in `RuntimeHintsAgentPlugin`

[sbrannen]

When running the Gradle build, we currently encounter the following warning.

> Task :buildSrc:compileJava
Note: /<...>/spring-framework/buildSrc/src/main/java/org/springframework/build/hint/RuntimeHintsAgentPlugin.java uses or overrides a deprecated API.

We should revise the implementation of RuntimeHintsAgentPlugin to resolve those warnings.

[bclozel]

Closing in favor of #34390