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cmake: fix warning about minimum required version

Since CMake 3.27 compatibility with versions of CMake older than
3.5 is deprecated [1]. CMake produces an annoying warning on the
configuration stage:

| CMake Deprecation Warning at src/CMakeLists.txt:7 (cmake_minimum_required):
|  Compatibility with CMake < 3.5 will be removed from a future version of
|  CMake.

We cannot bump a minimum required CMake version without bumping it in
the Tarantool build system. However, we can set a <policy_max>
(introduced in CMake 3.12, see [1]) and suppress a warning. <policy_max>
means that this CMake version is known to work properly and will not
result in any build errors right away for higher versions.

Note that the current CMake minimum required version in Tarantool is
equal to 2.8, but <policy_max> is introduced in CMake 3.12 [1].
However, according to [1] it is not a problem because if CMake is "older
than 3.12, the extra ... dots will be seen as version component
separators, resulting in the ...<max> part being ignored and preserving
the pre-3.12 behavior of basing policies on <min>".

<policy_max> is set to 3.18 because compatibility with versions of CMake
older than 2.8.12 is deprecated. Calls to
cmake_minimum_required(VERSION) that do not specify at least 2.8.12 as
their policy version (optionally via ...<max>) will produce a
deprecation warning in CMake 3.19 and above [2]. Compatibility with
2.8.12 is needed for CMP0002 [3], see commit 049e296ee114 ("test: run
LuaJIT tests via CMake").

1. https://cmake.org/cmake/help/latest/command/cmake_minimum_required.html
2. https://cmake.org/cmake/help/latest/command/cmake_minimum_required.html#policy-settings
3. https://cmake.org/cmake/help/latest/policy/CMP0002.html

Reviewed-by: Maxim Kokryashkin <m.kokryashkin@tarantool.org>
Reviewed-by: Sergey Kaplun <skaplun@tarantool.org>
Signed-off-by: Sergey Kaplun <skaplun@tarantool.org>

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