bump stage0 to production 1.52.0

Author: pietroalbini

src/stage0.txt

@@ -12,7 +12,7 @@
 # stable release's version number. `date` is the date where the release we're
 # bootstrapping off was released.
 
-date: 2021-05-04
+date: 2021-05-06
 rustc: 1.52.0
 
 # We use a nightly rustfmt to format the source because it solves some
@@ -39,4 +39,4 @@ rustc: 1.52.0
 # looking at a beta source tarball and it's uncommented we'll shortly comment it
 # out.
 
-dev: 1
+#dev: 1