Switch away from deprecated CI images (#552)

* ci: Upgrade from macos-10.15 to macos-11

The macos-10.15 CI image has been deprecated by Github since 2022-05-31, fully unsupported since
2022-08-30, and is slated to be fully removed by 2023-03-31 [1].

Update the relevant jobs to macOS 11, which is the oldest macOS version that still has a supported
image.

[1] https://github.blog/changelog/2022-07-20-github-actions-the-macos-10-15-actions-runner-image-is-being-deprecated-and-will-be-removed-by-8-30-22/

* ci: Upgrade from ubuntu-18.04 to ubuntu-20.04

The ubuntus-18.04 CI image has been deprecated by Github since 2022-08-08 and will be fully
unsupported by 2023-04-01 [1].

Update the relevant jobs to Ubuntu 20.04, which is the oldest Ubuntu version that still has a
supported image.

[1] https://github.blog/changelog/2022-08-09-github-actions-the-ubuntu-18-04-actions-runner-image-is-being-deprecated-and-will-be-removed-by-12-1-22/

Author: LingMan

.github/workflows/main.yml

@@ -216,13 +216,13 @@ jobs:
       QEMU_BUILD_VERSION: 7.0.0
     strategy:
       matrix:
-        build: [ubuntu, ubuntu-18.04, i686-linux, aarch64-linux, powerpc64le-linux, riscv64-linux, s390x-linux, arm-linux, ubuntu-stable, ubuntu-1.48, i686-linux-stable, aarch64-linux-stable, riscv64-linux-stable, s390x-linux-stable, mipsel-linux-stable, mips64el-linux-stable, powerpc64le-linux-stable, arm-linux-stable, ubuntu-1.48, i686-linux-1.48, aarch64-linux-1.48, riscv64-linux-1.48, s390x-linux-1.48, mipsel-linux-1.48, mips64el-linux-1.48, powerpc64le-linux-1.48, arm-linux-1.48, macos-latest, macos-10.15, windows, windows-2019]
+        build: [ubuntu, ubuntu-20.04, i686-linux, aarch64-linux, powerpc64le-linux, riscv64-linux, s390x-linux, arm-linux, ubuntu-stable, ubuntu-1.48, i686-linux-stable, aarch64-linux-stable, riscv64-linux-stable, s390x-linux-stable, mipsel-linux-stable, mips64el-linux-stable, powerpc64le-linux-stable, arm-linux-stable, ubuntu-1.48, i686-linux-1.48, aarch64-linux-1.48, riscv64-linux-1.48, s390x-linux-1.48, mipsel-linux-1.48, mips64el-linux-1.48, powerpc64le-linux-1.48, arm-linux-1.48, macos-latest, macos-11, windows, windows-2019]
         include:
           - build: ubuntu
             os: ubuntu-latest
             rust: nightly
-          - build: ubuntu-18.04
-            os: ubuntu-18.04
+          - build: ubuntu-20.04
+            os: ubuntu-20.04
             rust: nightly
           - build: i686-linux
             os: ubuntu-latest
@@ -443,8 +443,8 @@ jobs:
           - build: macos-latest
             os: macos-latest
             rust: stable
-          - build: macos-10.15
-            os: macos-10.15
+          - build: macos-11
+            os: macos-11
             rust: stable
           - build: windows
             os: windows-latest