Bug 1409533 - Require rust 1.21.0. r=froydnj

Per schedule, bump the minimum-supported rust version to 1.21.0
two weeks after its stable release so we can use new code which
depends on it.

MozReview-Commit-ID: Bn8UjvTC7uw

build/moz.configure/rust.configure

@@ -58,7 +58,7 @@ def rust_compiler(rustc_info, cargo_info):
         You can install rust by running './mach bootstrap'
         or by directly running the installer from https://rustup.rs/
         '''))
-    rustc_min_version = Version('1.20.0')
+    rustc_min_version = Version('1.21.0')
     cargo_min_version = Version('0.{}'.format(rustc_min_version.minor + 1))
 
     version = rustc_info.version

taskcluster/ci/build/linux.yml

@@ -180,7 +180,7 @@ linux64-base-toolchains/opt:
     toolchains:
         - linux64-clang-3.9
         - linux64-gcc-4.9
-        - linux64-rust-1.20
+        - linux64-rust-1.21
         - linux64-sccache
 
 linux64-base-toolchains/debug:
@@ -211,7 +211,7 @@ linux64-base-toolchains/debug:
     toolchains:
         - linux64-clang-3.9
         - linux64-gcc-4.9
-        - linux64-rust-1.20
+        - linux64-rust-1.21
         - linux64-sccache
 
 linux/opt:

taskcluster/ci/toolchain/linux.yml

@@ -323,30 +323,6 @@ linux64-rust-1.21:
         toolchain-alias: linux64-rust
         toolchain-artifact: public/build/rustc.tar.xz
 
-linux64-rust-1.20:
-    description: "rust repack"
-    treeherder:
-        kind: build
-        platform: toolchains/opt
-        symbol: TL(rust-1.20)
-        tier: 1
-    worker-type: aws-provisioner-v1/gecko-{level}-b-linux
-    worker:
-        docker-image: {in-tree: desktop-build}
-        max-run-time: 7200
-        env:
-          UPLOAD_DIR: artifacts
-    run:
-        using: toolchain-script
-        script: repack_rust.py
-        arguments: [
-            '--channel', '1.20.0',
-            '--host', 'x86_64-unknown-linux-gnu',
-            '--target', 'x86_64-unknown-linux-gnu',
-            '--target', 'i686-unknown-linux-gnu',
-        ]
-        toolchain-artifact: public/build/rustc.tar.xz
-
 linux64-rust-1.19:
     description: "rust repack"
     treeherder: