chore: regenerate README (googleapis#414)

This PR was generated using Autosynth. 🌈


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

```
2021-03-28 23:45:07,520 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-spanner-jdbc/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-03-28 23:45:08,722 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/91579ce3-d49c-4399-bfda-3caaae494359/targets

- [ ] To automatically regenerate this PR, check this box. (May take up to 24 hours.)

Author: yoshi-automation

java-spanner-jdbc/.github/readme/synth.metadata/synth.metadata

@@ -4,14 +4,14 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/java-spanner-jdbc.git",
-        "sha": "f0cd180f7a34c28460ff88699614cf1173205002"
+        "sha": "032f2c63dc95d4fc2bc4d05738031c1d1d880b4f"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "bb854b6c048619e3be4e8b8ce8ed10aa74ea78ef"
+        "sha": "60fc32cc950c20cfdd774b846a4a7dc5d1e4d0ef"
       }
     }
   ]

java-spanner-jdbc/README.md

@@ -108,7 +108,7 @@ To get help, follow the instructions in the [shared Troubleshooting document][tr
 
 ## Java Versions
 
-Java 7 or above is required for using this client.
+Java 8 or above is required for using this client.
 
 ## Versioning
 
@@ -135,7 +135,6 @@ Apache 2.0 - See [LICENSE][license] for more information.
 
 Java Version | Status
 ------------ | ------
-Java 7 | [![Kokoro CI][kokoro-badge-image-1]][kokoro-badge-link-1]
 Java 8 | [![Kokoro CI][kokoro-badge-image-2]][kokoro-badge-link-2]
 Java 8 OSX | [![Kokoro CI][kokoro-badge-image-3]][kokoro-badge-link-3]
 Java 8 Windows | [![Kokoro CI][kokoro-badge-image-4]][kokoro-badge-link-4]