chore: regenerate README (#464)

This PR was generated using Autosynth. 🌈


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

```
2021-06-23 16:03:07,319 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-datastore/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-06-23 16:03:08,693 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/27a9a463-6e36-4bed-a4d6-bf14509658d9/targets

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

@@ -4,14 +4,14 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/java-datastore.git",
-        "sha": "9ed656033f0fd21656ec72ceea215d3cf52cd5c4"
+        "sha": "147a3e5a80b2597258c4b78ae54d4be5d45b4df1"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "8f76a885deaaf2fe234daeba4a8cc4d1b3de8086"
+        "sha": "9763f20e4b7bb1091082462b2f7970e965d0d414"
       }
     }
   ]

README.md

@@ -45,7 +45,7 @@ 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:20.6.0')
+implementation platform('com.google.cloud:libraries-bom:20.7.0')
 
 compile 'com.google.cloud:google-cloud-datastore'
 ```