add backward-cpp/1.6

[AndreyMlashkin]

Specify library name and version: backward-cpp/1.6

This is also a good place to share with all of us why you are submitting this PR (specially if it is a new addition to ConanCenter): is it a dependency of other libraries you want to package? Are you the author of the library? Thanks!

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• I've read the guidelines for contributing.
• I've followed the PEP8 style guides for Python code in the recipes.
• I've used the latest Conan client version.
• I've tried at least one configuration locally with the
  conan-center hook activated.

[AndreyMlashkin]

Is there an easy possibility to check if compiler supports cpp17?

[prince-chrismc]

Is there an easy possibility to check if compiler supports cpp17?

conan-io/conan#8002 is my go to

[conan-center-bot]

All green in build 4 (f3bce82122abc775968f0a4b850dc9eb35719acb):

• backward-cpp/1.4@:
  All packages built successfully! (All logs)

• backward-cpp/1.6@:
  All packages built successfully! (All logs)

• backward-cpp/1.5@:
  All packages built successfully! (All logs)