Improve jetcd and grpc dependency

[sandynz]

Changes proposed in this pull request:

• Upgrade jetcd dependency version from 0.7.5 to 0.7.6. But it still reference old version of grpc-protobuf (which will be upgraded to 1.58.0 without vulnerabilities)
• Replace grpc-all dependency to separated ones to reduce unnecessary dependencies

grpc-all includes too much dependencies with compile scope, e.g. grpc-testing which depends on junit on compile scope, and it'll be removed from dependencies.

<dependency>
<groupId>io.grpc</groupId>
<artifactId>grpc-testing</artifactId>
<version>1.58.0</version>
<scope>compile</scope>

Simplified dependency chain: grpc-xxx <- jetcd-grpc <- jetcd-core.

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