chore: regenerate README (#487)

This PR was generated using Autosynth. 🌈


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

```
2021-08-11 22:34:42,550 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-logging-logback/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-08-11 22:34:43,911 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/a1e9c6c9-c3f6-46c4-9fcb-1b5873f9764c/targets

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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": "09ee009512d593065507d92092c7531f5af2d379"
+        "sha": "1c20e4c15f095ce7078959dda79b10dd16cc0687"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "df7fc1e3a6df4316920ab221431945cdf9aa7217"
+        "sha": "7e1f6da50524b5d98eb67adbf6dd0805df54233d"
       }
     }
   ]

README.md

@@ -22,20 +22,20 @@ 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>0.121.5-alpha</version>
+  <version>0.121.6-alpha</version>
 </dependency>
 ```
 
 If you are using Gradle without BOM, add this to your dependencies
 
 ```Groovy
-compile 'com.google.cloud:google-cloud-logging-logback:0.121.5-alpha'
+compile 'com.google.cloud:google-cloud-logging-logback:0.121.6-alpha'
 ```
 
 If you are using SBT, add this to your dependencies
 
 ```Scala
-libraryDependencies += "com.google.cloud" % "google-cloud-logging-logback" % "0.121.5-alpha"
+libraryDependencies += "com.google.cloud" % "google-cloud-logging-logback" % "0.121.6-alpha"
 ```
 
 ## Authentication