Don't allow cortex-m-rt <= 0.6.15

rmsc · rmsc · commit 17b1a71dbc13 · 2021-08-27T19:07:08.000+01:00
Previous versions didn't set the 'links' cargo field, and risk being
pulled together with newer ones.
diff --git a/scripts/makecrates.py b/scripts/makecrates.py
@@ -78,7 +78,7 @@
 
 [dependencies.cortex-m-rt]
 optional = true
-version = ">=0.6.13,<0.8"
+version = ">=0.6.15,<0.8"
 
 [package.metadata.docs.rs]
 features = {docs_features}
