Bump cmake_minimum_required to 3.5 for CMake 4.x compatibility

[davefx]

Summary

CMake 4.x has removed compatibility with cmake_minimum_required values below 3.5, so configuring grive2 with a modern CMake (e.g. on Ubuntu 26.04, which ships CMake 4.2) fails with:

CMake Error at CMakeLists.txt:1 (cmake_minimum_required):
  Compatibility with CMake < 3.5 has been removed from CMake.

  Update the VERSION argument <min> value.  Or, use the <min>...<max> syntax
  to tell CMake that the project requires at least <min> but has been updated
  to work with policies introduced by <max> or earlier.

This bumps the minimum required version to 3.5 so the project configures cleanly under CMake 4.x while still supporting older CMake releases (3.5 was released in 2016).

Test plan

• cmake configures successfully on Ubuntu 26.04 with CMake 4.2.3
• make builds grive, libgrive.a, btest, and unittest to completion
• Resulting binary reports grive version 0.5.3

Note: a fully clean build on Ubuntu 26.04 also requires the changes from #403 (libgcrypt pkg-config fallback) and #410 (Boost 1.89+ no longer ships boost::system as a separate library). This PR intentionally only addresses the CMake version bump to keep it focused.