[BUILD] Bump CMake minimum required version to 3.9

[marcalff]

Fixes #1399

Also resolves the following deprecation warnings:

Starting with CMake 3.27, support for CMake < 3.5 is deprecated.

Changed in version 3.27: Compatibility with versions of CMake older than 3.5 is deprecated.

For some platforms, CI logs for opentelemetry-cpp now shows:

CMake Deprecation Warning at CMakeLists.txt:4 (cmake_minimum_required):
  Compatibility with CMake < 3.5 will be removed from a future version of
  CMake.

Changes

Please provide a brief description of the changes here.

• Bump cmake_minimum_required to 3.9
  • at least 3.5 is required to clear the deprecation warning
  • at least 3.9 is required because gtest is used, the previous requirement of 3.1 was incorrect.

For significant contributions please make sure you have completed the following items:

• CHANGELOG.md updated for non-trivial changes
• Unit tests have been added
• Changes in public API reviewed

[esigo]

LGTM
Thanks :)

[owent]

LGTM and thanks.