chore: regenerate README (#552)

This PR was generated using Autosynth. 🌈


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

```
2021-06-02 20:17:56,102 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-errorreporting/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-06-02 20:17:57,224 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/1cde6a7c-dd24-49f1-92c4-0142c19a4a01/targets

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

@@ -4,14 +4,14 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/java-errorreporting.git",
-        "sha": "158bf16a2c74e5fdae7829e7716a3580cf90a3d1"
+        "sha": "bfbebeccad3196480e65b9562bca9d8a9a0a17dc"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "1778de119522ea9c9b18763c2a4577e22a60433c"
+        "sha": "c3e41da0fa256ad7f6b4bc76b9d069dedecdfef4"
       }
     }
   ]

README.md

@@ -60,7 +60,7 @@ If you are using Maven without BOM, add this to your dependencies:
 <dependency>
   <groupId>com.google.cloud</groupId>
   <artifactId>google-cloud-errorreporting</artifactId>
-  <version>0.120.42-beta</version>
+  <version>0.121.0-beta</version>
 </dependency>
 
 ```
@@ -73,12 +73,12 @@ compile 'com.google.cloud:google-cloud-errorreporting'
 ```
 If you are using Gradle without BOM, add this to your dependencies
 ```Groovy
-compile 'com.google.cloud:google-cloud-errorreporting:0.120.42-beta'
+compile 'com.google.cloud:google-cloud-errorreporting:0.121.0-beta'
 ```
 
 If you are using SBT, add this to your dependencies
 ```Scala
-libraryDependencies += "com.google.cloud" % "google-cloud-errorreporting" % "0.120.42-beta"
+libraryDependencies += "com.google.cloud" % "google-cloud-errorreporting" % "0.121.0-beta"
 ```
 
 ## Authentication