Bump io.grpc:grpc-api from 1.57.2 to 1.68.0 in /plugins/discovery-gce

Bumps [io.grpc:grpc-api](https://github.com/grpc/grpc-java) from 1.57.2 to 1.68.0.
- [Release notes](https://github.com/grpc/grpc-java/releases)
- [Commits](https://github.com/grpc/grpc-java/commits)

---
updated-dependencies:
- dependency-name: io.grpc:grpc-api
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>

plugins/discovery-gce/build.gradle

@@ -30,7 +30,7 @@ dependencies {
   api "commons-logging:commons-logging:${versions.commonslogging}"
   api "org.apache.logging.log4j:log4j-1.2-api:${versions.log4j}"
   api "commons-codec:commons-codec:${versions.commonscodec}"
-  api 'io.grpc:grpc-api:1.57.2'
+  api 'io.grpc:grpc-api:1.68.0'
   api 'io.opencensus:opencensus-api:0.31.1'
   api 'io.opencensus:opencensus-contrib-http-util:0.31.1'
   runtimeOnly "com.google.guava:guava:${versions.guava}"