chore: regenerate README (#1301)

This PR was generated using Autosynth. 🌈


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

```
2021-07-19 01:28:42,483 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-spanner/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-07-19 01:28:43,808 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/d70282dd-aa9f-4052-bb27-582585f83d43/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-spanner.git",
-        "sha": "5a045e091e26b878a0765e6924847215ba3a545f"
+        "sha": "d72c2f79f8cf0b83da00060587a079ce859c87a2"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "09c59c20a4bf0daed1665af59035ff240fe356df"
+        "sha": "4124d245133c0f740ac5e6fa2e7daeba327f1297"
       }
     }
   ]

README.md

@@ -260,10 +260,50 @@ To get help, follow the instructions in the [shared Troubleshooting document][tr
 
 Cloud Spanner uses gRPC for the transport layer.
 
-## Java Versions
+## Supported Java Versions
 
 Java 8 or above is required for using this client.
 
+Google's Java client libraries,
+[Google Cloud Client Libraries][cloudlibs]
+and
+[Google Cloud API Libraries][apilibs],
+follow the
+[Oracle Java SE support roadmap][oracle]
+(see the Oracle Java SE Product Releases section).
+
+### For new development
+
+In general, new feature development occurs with support for the lowest Java
+LTS version covered by  Oracle's Premier Support (which typically lasts 5 years
+from initial General Availability). If the minimum required JVM for a given
+library is changed, it is accompanied by a [semver][semver] major release.
+
+Java 11 and (in September 2021) Java 17 are the best choices for new
+development.
+
+### Keeping production systems current
+
+Google tests its client libraries with all current LTS versions covered by
+Oracle's Extended Support (which typically lasts 8 years from initial
+General Availability).
+
+#### Legacy support
+
+Google's client libraries support legacy versions of Java runtimes with long
+term stable libraries that don't receive feature updates on a best efforts basis
+as it may not be possible to backport all patches.
+
+Google provides updates on a best efforts basis to apps that continue to use
+Java 7, though apps might need to upgrade to current versions of the library
+that supports their JVM.
+
+#### Where to find specific information
+
+The latest versions and the supported Java versions are identified on
+the individual GitHub repository `github.com/GoogleAPIs/java-SERVICENAME`
+and on [google-cloud-java][g-c-j].
+
 ## Versioning
 
 
@@ -323,3 +363,9 @@ Java is a registered trademark of Oracle and/or its affiliates.
 [enable-api]: https://console.cloud.google.com/flows/enableapi?apiid=spanner.googleapis.com
 [libraries-bom]: https://github.com/GoogleCloudPlatform/cloud-opensource-java/wiki/The-Google-Cloud-Platform-Libraries-BOM
 [shell_img]: https://gstatic.com/cloudssh/images/open-btn.png
+
+[semver]: https://semver.org/
+[cloudlibs]: https://cloud.google.com/apis/docs/client-libraries-explained
+[apilibs]: https://cloud.google.com/apis/docs/client-libraries-explained#google_api_client_libraries
+[oracle]: https://www.oracle.com/java/technologies/java-se-support-roadmap.html
+[g-c-j]: http://github.com/googleapis/google-cloud-java