ci: move to macos-11

GitHub has deprecated macOS 10.15; move to their new macOS 11 build
servers.

.github/workflows/benchmark.yml

@@ -27,7 +27,7 @@ jobs:
           os: ubuntu-latest
           setup-script: ubuntu
         - name: "macOS"
-          os: macos-10.15
+          os: macos-11
           env:
             CC: clang
             CMAKE_OPTIONS: -DREGEX_BACKEND=regcomp_l -DDEPRECATE_HARD=ON -DUSE_GSSAPI=ON -DBUILD_TESTS=OFF -DBUILD_EXAMPLES=OFF -DBUILD_CLI=ON -DCMAKE_BUILD_TYPE=Release

.github/workflows/main.yml

@@ -122,7 +122,7 @@ jobs:
           os: ubuntu-latest
         - name: "macOS"
           id: macos
-          os: macos-10.15
+          os: macos-11
           env:
             CC: clang
             CMAKE_OPTIONS: -DREGEX_BACKEND=regcomp_l -DDEPRECATE_HARD=ON -DUSE_LEAK_CHECKER=leaks -DUSE_GSSAPI=ON
@@ -230,7 +230,7 @@ jobs:
           os: ubuntu-latest
         - name: "macOS (SHA256)"
           id: macos
-          os: macos-10.15
+          os: macos-11
           env:
             CC: clang
             CMAKE_OPTIONS: -DREGEX_BACKEND=regcomp_l -DDEPRECATE_HARD=ON -DUSE_LEAK_CHECKER=leaks -DUSE_GSSAPI=ON -DEXPERIMENTAL_SHA256=ON

.github/workflows/nightly.yml

@@ -154,7 +154,7 @@ jobs:
             SKIP_SSH_TESTS: true
           os: ubuntu-latest
         - name: "macOS"
-          os: macos-10.15
+          os: macos-11
           env:
             CC: clang
             CMAKE_OPTIONS: -DREGEX_BACKEND=regcomp_l -DDEPRECATE_HARD=ON -DUSE_LEAK_CHECKER=leaks -DUSE_GSSAPI=ON