ci: R.I.P. Travis CI - Long live GitHub Actions

Follows:
- boostorg#544

Outstanding tasks:
- boostorg#549
- boostorg#548

(cherry picked from develop commit 434e78f)

ci: R.I.P. Travis CI - Long live GitHub Actions

Follows:
- boostorg#544

Outstanding tasks:
- boostorg#549
- boostorg#548

(cherry picked from develop commit 434e78f)

ci: R.I.P. Travis CI - Long live GitHub Actions

Follows:
- boostorg#544

Outstanding tasks:
- boostorg#549
- boostorg#548

(cherry picked from develop commit 434e78f)

ci: R.I.P. Travis CI - Long live GitHub Actions

Follows:
- boostorg#544

Outstanding tasks:
- boostorg#549
- boostorg#548

(cherry picked from develop commit 434e78f)

ci: R.I.P. Travis CI - Long live GitHub Actions

Follows:
- boostorg#544

Outstanding tasks:
- boostorg#549
- boostorg#548

(cherry picked from develop commit 434e78f)

ci: R.I.P. Travis CI - Long live GitHub Actions

Follows:
- boostorg#544

Outstanding tasks:
- boostorg#549
- boostorg#548

(cherry picked from develop commit 434e78f)

Third merge of develop branch for Boost 1.74 release

* origin/develop:
  README.md: Update link to Azure Pipelines builds [ci skip]
  Fix dynamic image apply_operation when building with -std=c++11 (boostorg#511)

Third merge of develop branch for Boost 1.74 release

* origin/develop:
  README.md: Update link to Azure Pipelines builds [ci skip]
  Fix dynamic image apply_operation when building with -std=c++11 (boostorg#511)

Third merge of develop branch for Boost 1.74 release

* origin/develop:
  README.md: Update link to Azure Pipelines builds [ci skip]
  Fix dynamic image apply_operation when building with -std=c++11 (boostorg#511)

Initial merge of develop branch for Boost 1.74 release

* develop:
  Collect release notes for Boost 1.74
  Fix missing header in numeric/kernel.hpp to make it self-contained (boostorg#502)
  Use perfect forwading from apply_operation to visit (boostorg#491)
  Fix typos and replace mpl with mp11 in tutorial (boostorg#494)
  Implemented mechanism to reverse kernel_2d (boostorg#489)
  added missing const& in extend_boundary parameters (boostorg#490)
  Add initializing image constructor (boostorg#486)
  Fix interleaved_view factory using point<std::ptrdiff_t> for dimension (boostorg#487)
  RELEASES.md: Remove beta tag [ci skip]
  Replace Boost.Variant with Boost.Variant2 (boostorg#474)
  Fix error plane_view_t is not a class or namespace name (boostorg#481)
  Fix image constructor from other image (boostorg#477)
  Fix overflow in RGB to CMYK32 conversion (boostorg#470)