Bump version to v7.1.0-beta.0 (not a release) (#7269)

Having merged the drop-headtracker PR we now have a DB schema change in `unstable` compared to `release-v7.0.0`:

- https://github.com/sigp/lighthouse/pull/6744

There is a DB downgrade available, however this needs to be applied manually and it's usually a bit of a hassle.

This PR bumps the version on `unstable` to `v7.1.0-beta.0` _without_ actually cutting a `v7.1.0-beta.0` release, so that we can tell at a glance which schema version a node is using.

common/lighthouse_version/src/lib.rs

@@ -17,8 +17,8 @@ pub const VERSION: &str = git_version!(
         // NOTE: using --match instead of --exclude for compatibility with old Git
         "--match=thiswillnevermatchlol"
     ],
-    prefix = "Lighthouse/v7.0.0-beta.5-",
-    fallback = "Lighthouse/v7.0.0-beta.5"
+    prefix = "Lighthouse/v7.1.0-beta.0-",
+    fallback = "Lighthouse/v7.1.0-beta.0"
 );
 
 /// Returns the first eight characters of the latest commit hash for this build.
@@ -54,7 +54,7 @@ pub fn version_with_platform() -> String {
 ///
 /// `1.5.1`
 pub fn version() -> &'static str {
-    "7.0.0-beta.5"
+    "7.1.0-beta.0"
 }
 
 /// Returns the name of the current client running.