ci: replace macOS 10.15 with macOS 11 in CI pipelines (#717)

* ci: replace macOS 10.15 with macOS 11 in CI workflow

GitHub started the deprecation process for macOS 10.15 runners
on 2022-05-31, and macOS 10.15 will be completely unsupported by
2022-08-30 (if things proceed as planned).

See <actions/runner-images#5583> for
more information on the removal of the macOS 10.15 images.

This issue already hit us during the build for PR #716, because
at that time there was a scheduled brownout for macOS 10.15
builds to raise awareness of the upcoming removal.

* ci: remove Azure pipeline build configuration

See <#717 (comment)>.

.github/workflows/ci.yml

@@ -79,7 +79,7 @@ jobs:
             os: ubuntu-20.04
           - toolset: clang
             cxxstd: "14,17"
-            os: macos-10.15
+            os: macos-11
 
     runs-on: ${{matrix.os}}
 

README.md

@@ -10,10 +10,10 @@
 [![Conan](https://img.shields.io/badge/on-conan-blue.svg)](https://bintray.com/bincrafters/public-conan/boost_gil%3Abincrafters)
 [![Vcpkg](https://img.shields.io/badge/on-vcpkg-blue.svg)](https://github.com/Microsoft/vcpkg/tree/master/ports/boost-gil)
 
-Documentation | GitHub Actions | AppVeyor | Azure Pipelines | Regression | Codecov
---------------|----------------|----------|-----------------|------------|----------
-[![develop](https://img.shields.io/badge/doc-develop-blue.svg)](https://boostorg.github.io/gil/develop/) | [![GitHub Actions](https://github.com/boostorg/gil/workflows/CI/badge.svg?branch=develop)](https://github.com/boostorg/gil/actions?query=branch:develop) | [![AppVeyor](https://ci.appveyor.com/api/projects/status/w4k19d8io2af168h/branch/develop?svg=true)](https://ci.appveyor.com/project/stefanseefeld/gil/branch/develop) | [![Azure](https://dev.azure.com/boostorg/gil/_apis/build/status/boostorg.gil?branchName=develop)](https://dev.azure.com/boostorg/gil/_build/latest?definitionId=7&branchName=develop) | [![gil](https://img.shields.io/badge/gil-develop-blue.svg)](http://www.boost.org/development/tests/develop/developer/gil.html) | [![codecov](https://codecov.io/gh/boostorg/gil/branch/develop/graphs/badge.svg)](https://app.codecov.io/gh/boostorg/gil/branch/develop)
-[![master](https://img.shields.io/badge/doc-master-blue.svg)](https://boostorg.github.io/gil/) | [![GitHub Actions](https://github.com/boostorg/gil/workflows/CI/badge.svg?branch=master)](https://github.com/boostorg/gil/actions?query=branch:master) | [![AppVeyor](https://ci.appveyor.com/api/projects/status/w4k19d8io2af168h?svg=true)](https://ci.appveyor.com/project/stefanseefeld/gil/branch/master) | [![Azure](https://dev.azure.com/boostorg/gil/_apis/build/status/boostorg.gil?branchName=master)](https://dev.azure.com/boostorg/gil/_build/latest?definitionId=7&branchName=master) | [![gil](https://img.shields.io/badge/gil-master-blue.svg)](http://www.boost.org/development/tests/master/developer/gil.html) | [![codecov](https://codecov.io/gh/boostorg/gil/branch/master/graphs/badge.svg)](https://app.codecov.io/gh/boostorg/gil/branch/master)
+Documentation | GitHub Actions | AppVeyor | Regression | Codecov
+--------------|----------------|----------|------------|----------
+[![develop](https://img.shields.io/badge/doc-develop-blue.svg)](https://boostorg.github.io/gil/develop/) | [![GitHub Actions](https://github.com/boostorg/gil/workflows/CI/badge.svg?branch=develop)](https://github.com/boostorg/gil/actions?query=branch:develop) | [![AppVeyor](https://ci.appveyor.com/api/projects/status/w4k19d8io2af168h/branch/develop?svg=true)](https://ci.appveyor.com/project/stefanseefeld/gil/branch/develop) | [![gil](https://img.shields.io/badge/gil-develop-blue.svg)](http://www.boost.org/development/tests/develop/developer/gil.html) | [![codecov](https://codecov.io/gh/boostorg/gil/branch/develop/graphs/badge.svg)](https://app.codecov.io/gh/boostorg/gil/branch/develop)
+[![master](https://img.shields.io/badge/doc-master-blue.svg)](https://boostorg.github.io/gil/) | [![GitHub Actions](https://github.com/boostorg/gil/workflows/CI/badge.svg?branch=master)](https://github.com/boostorg/gil/actions?query=branch:master) | [![AppVeyor](https://ci.appveyor.com/api/projects/status/w4k19d8io2af168h?svg=true)](https://ci.appveyor.com/project/stefanseefeld/gil/branch/master) | [![gil](https://img.shields.io/badge/gil-master-blue.svg)](http://www.boost.org/development/tests/master/developer/gil.html) | [![codecov](https://codecov.io/gh/boostorg/gil/branch/master/graphs/badge.svg)](https://app.codecov.io/gh/boostorg/gil/branch/master)
 
 # Boost.GIL