Bump cmake minimum version to 3.12

Following a change in libmariadb 3.4 with CONC-717:

a19895efff5060531428dbb19803298d55b153bd

ed25519_ref10 was added as an OBJECT library, to be later linked to client_ed25519 with cmake's target_link_libraries.

Prior to CMake 3.12, one can not link OBJECT libraries directly and we get this error during configuration:

CMake Error at libmariadb/cmake/plugins.cmake:71 (target_link_libraries):

  Target "ed25519_ref10" of type OBJECT_LIBRARY may not be linked into

  another target.  One may link only to STATIC or SHARED libraries, or to

  executables with the ENABLE_EXPORTS property set.

Call Stack (most recent call first):

  libmariadb/plugins/auth/CMakeLists.txt:74 (REGISTER_PLUGIN)

  libmariadb/plugins/CMakeLists.txt:7 (INCLUDE)

  libmariadb/CMakeLists.txt:447 (INCLUDE)

See the addition of this feature in CMake's documentation.

Given that libmariadb has introduced a requirement for CMake 3.12 and that we do not have any platforms still supported that feature an older CMake (RHEL7 comes with cmake 3.17 in epel), it should be safe to bump the minimum Server (and libmariadb) cmake requirement to 3.12.