chore: regenerate README (#429)

This PR was generated using Autosynth. 🌈


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

```
2021-06-02 20:16:24,056 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-logging-logback/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-06-02 20:16:25,227 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/ec7f36fd-002b-46fc-b0bc-fc670b1f0f18/targets

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

Author: yoshi-automation

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

@@ -4,14 +4,14 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/java-logging-logback.git",
-        "sha": "2ff2886c11a13c223a5c7b114ccb7c7d189c41dd"
+        "sha": "64b5c2b1592a1f22360c889661331453ebb2307c"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "7332178a11ddddc91188dc0f25bca1ccadcaa6c6"
+        "sha": "c3e41da0fa256ad7f6b4bc76b9d069dedecdfef4"
       }
     }
   ]

README.md

@@ -20,18 +20,18 @@ If you are using Maven, add this to your pom.xml file:
 <dependency>
   <groupId>com.google.cloud</groupId>
   <artifactId>google-cloud-logging-logback</artifactId>
-  <version>0.120.8-alpha</version>
+  <version>0.121.0-alpha</version>
 </dependency>
 ```
 
 If you are using Gradle without BOM, add this to your dependencies
 ```Groovy
-compile 'com.google.cloud:google-cloud-logging-logback:0.120.8-alpha'
+compile 'com.google.cloud:google-cloud-logging-logback:0.121.0-alpha'
 ```
 
 If you are using SBT, add this to your dependencies
 ```Scala
-libraryDependencies += "com.google.cloud" % "google-cloud-logging-logback" % "0.120.8-alpha"
+libraryDependencies += "com.google.cloud" % "google-cloud-logging-logback" % "0.121.0-alpha"
 ```
 
 ## Authentication