Fix Github workflows

* Ubuntu 18.04 GCC does not fully support C++17, hence remove the
  runner.
* Using CMake SuiteSparse in a C++17 project requires a workaround
  implemented in a recent release.

Change-Id: I9985fe12d582dfc9b74e97d670828334e507e9f5

.github/workflows/linux.yml

diff --git a/.github/workflows/linux.yml b/.github/workflows/linux.yml
index 75307b3..034fa9f 100644
--- a/.github/workflows/linux.yml
+++ b/.github/workflows/linux.yml
@@ -17,7 +17,6 @@
       fail-fast: true
       matrix:
         os:
-          - ubuntu-18.04
           - ubuntu-20.04
         build_type:
           - Release

.github/workflows/windows.yml

diff --git a/.github/workflows/windows.yml b/.github/workflows/windows.yml
index b8ff056..374fa94 100644
--- a/.github/workflows/windows.yml
+++ b/.github/workflows/windows.yml
@@ -92,12 +92,12 @@
         uses: actions/cache@v2
         with:
           path: suitesparse/
-          key: ${{matrix.msvc}}-suitesparse-5.10.1-cmake.1-${{matrix.arch}}-${{matrix.build_type}}-${{matrix.lib}}
+          key: ${{matrix.msvc}}-suitesparse-5.11.0-cmake.2-${{matrix.arch}}-${{matrix.build_type}}-${{matrix.lib}}
 
       - name: Download SuiteSparse
         if: steps.cache-suitesparse.outputs.cache-hit != 'true'
         run: |
-          (New-Object System.Net.WebClient).DownloadFile("https://github.com/sergiud/SuiteSparse/releases/download/5.10.1-cmake.1/SuiteSparse-5.10.1-cmake.1-${{matrix.marker}}-Win64-${{matrix.build_type}}-${{matrix.lib}}-gpl.zip", "suitesparse.zip");
+          (New-Object System.Net.WebClient).DownloadFile("https://github.com/sergiud/SuiteSparse/releases/download/5.11.0-cmake.2/SuiteSparse-5.11.0-cmake.2-${{matrix.marker}}-Win64-${{matrix.build_type}}-${{matrix.lib}}-gpl.zip", "suitesparse.zip");
           Expand-Archive -Path suitesparse.zip -DestinationPath ${{github.workspace}}/suitesparse;
 
       - name: Cache Eigen