ci(release): Add .version file (#751)

### Changes

This PR adds a `.version` file as part of our updated release process.
It also updates `.shiprc` to ensure the new file is bumped during
releases.

### References

N/A

### Testing

N/A

### Contributor Checklist

- [x] I agree to adhere to the [Auth0 General Contribution
Guidelines](https://github.com/auth0/open-source-template/blob/master/GENERAL-CONTRIBUTING.md).
- [x] I agree to uphold the [Auth0 Code of
Conduct](https://github.com/auth0/open-source-template/blob/master/CODE-OF-CONDUCT.md).

.shiprc

@@ -1,6 +1,7 @@
 {
   "files": {
-    "src/Auth0.php": []
+    "src/Auth0.php": [],
+    ".version": []
   },
   "prefixVersion": false
 }

.version

@@ -0,0 +1 @@
+8.10.0