chore: regenerate README (#475)

This PR was generated using Autosynth. 🌈


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

```
2021-06-23 19:15:51,536 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-iamcredentials/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-06-23 19:15:52,743 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/ca1abcae-0b5e-4bda-8523-02c61e2c7354/targets

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

@@ -4,7 +4,7 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/java-iamcredentials.git",
-        "sha": "4b584c985b8b2ac6d269ce7fba28f1ad9199df5c"
+        "sha": "e99c898f6ce30d7a21008b590c9c6cc833ebf07d"
       }
     },
     {

README.md

@@ -17,7 +17,7 @@ If you are using Maven with [BOM][libraries-bom], add this to your pom.xml file
     <dependency>
       <groupId>com.google.cloud</groupId>
       <artifactId>libraries-bom</artifactId>
-      <version>20.6.0</version>
+      <version>20.7.0</version>
       <type>pom</type>
       <scope>import</scope>
     </dependency>