servo: Merge #18883 - Update stable Rust version to 1.20.0 (from servo:1.20); r=SimonSapin

Source-Repo: https://github.com/servo/servo
Source-Revision: 4375a89f35424fa92b68a003531cadc811481532

--HG--
extra : subtree_source : https%3A//hg.mozilla.org/projects/converted-servo-linear
extra : subtree_revision : ec7d1357338119bfd314eb5f6b8e5d7ba6212e12

servo/rust-stable-version

@@ -1 +1 @@
-1.19.0
+1.20.0