Deploy 0.21 docs and drop 0.15 docs

Author: sdroege

.github/workflows/docs.yml

@@ -16,12 +16,12 @@ jobs:
     name: build
     env:
       RELEASES: |
+        0.21=0.21
         0.20=0.20
         0.19=0.19
         0.18=0.18
         0.17=0.17
         0.16=0.16
-        0.15=0.15
       RUSTFLAGS: --cfg docsrs
       RUSTDOCFLAGS: --cfg docsrs -Zunstable-options --generate-link-to-definition
     steps: