macos-latest for GitHub workflows (#626)

Workflows stopped running since we were on old MacOS versions. Using
latest should fix this longer-term.

.github/workflows/boards_build.yml

@@ -12,7 +12,7 @@ jobs:
   build_boards:
     strategy:
       matrix:
-        os: [ubuntu-latest, macos-10.15]
+        os: [ubuntu-latest, macos-latest]
     runs-on: ${{ matrix.os }}
     steps:
       - uses: actions/checkout@v2

.github/workflows/opensk_build.yml

@@ -9,7 +9,7 @@ jobs:
   build_ctap2:
     strategy:
       matrix:
-        os: [ubuntu-latest, macos-10.15]
+        os: [ubuntu-latest, macos-latest]
     runs-on: ${{ matrix.os }}
     steps:
       - uses: actions/checkout@v2

.github/workflows/reproducible.yml

@@ -9,7 +9,7 @@ jobs:
   check_hashes:
     strategy:
       matrix:
-        os: [ubuntu-latest, macos-10.15]
+        os: [ubuntu-latest, macos-latest]
       fail-fast: false
     runs-on: ${{ matrix.os }}
     steps: