chore: regenerate README (#794)

This PR was generated using Autosynth. 🌈


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

```
2021-08-16 22:23:25,398 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-pubsub/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-08-16 22:23:26,623 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/a9eebc21-71d1-4f11-80cf-3d5e3e37b8f1/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-pubsub.git",
-        "sha": "7940378db1394e3c5631ec854fa57b35cfb12094"
+        "sha": "4d1d1e7e93511c093eeb7f0e553058035a54b6fb"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "7e1f6da50524b5d98eb67adbf6dd0805df54233d"
+        "sha": "a01e1a0bf70754f51450958b966bd673945d1e6e"
       }
     }
   ]

README.md

@@ -51,7 +51,7 @@ If you are using Maven without BOM, add this to your dependencies:
 If you are using Gradle 5.x or later, add this to your dependencies
 
 ```Groovy
-implementation platform('com.google.cloud:libraries-bom:20.9.0')
+implementation platform('com.google.cloud:libraries-bom:21.0.0')
 
 compile 'com.google.cloud:google-cloud-pubsub'
 ```