chore: regenerate README (#129)

This PR was generated using Autosynth. 🌈


<details><summary>Log from Synthtool</summary>

```
2021-07-07 17:14:22,393 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-channel/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-07-07 17:14:23,712 synthtool [DEBUG] > Wrote metadata to .github/readme/synth.metadata/synth.metadata.

```
</details>

Full log will be available here:
https://source.cloud.google.com/results/invocations/f9d10483-1546-497e-ac78-ed9a93753239/targets

- [ ] To automatically regenerate this PR, check this box. (May take up to 24 hours.)

.github/readme/synth.metadata/synth.metadata

@@ -4,7 +4,7 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/java-channel.git",
-        "sha": "3736b42ee5a65bb2639786df6a536dda7cda4a51"
+        "sha": "f9b923cc5a9e0f61fcdb49071b2c007ad1a55e94"
       }
     },
     {

README.md

@@ -17,7 +17,7 @@ If you are using Maven, add this to your pom.xml file:
 <dependency>
   <groupId>com.google.cloud</groupId>
   <artifactId>google-cloud-channel</artifactId>
-  <version>2.2.1</version>
+  <version>2.3.0</version>
 </dependency>
 
 ```