crud uses tuple-merger for select. Built-in merger is provided for the following versions

and external merger can be provided for the following versions

There is a separate select implementation without merger for older versions without internal merger support (<= 1.10.7, == 2.5.0, >= 2.4.0, <= 2.4.1, >= 2, <= 2.3.2) and without internal merger if external not provided (< 2.0.0): select_old.

All versions that require select_old are out of support for a long time: it covers <= 1.10.7 and >=2, <= 2.5.2 releases. There is also the >= 1.10.8, <= 1.10.15version range withselect_old` support.

Supporting two select implementation is really bothersome. A lot of time spent to update a code which doesn't seems relevant anymore. I propose to remove this code.

The course of action is as follows.

1. Ensure that it is fine to drop support of Tarantool releases <= 1.10.7 >=2, <= 2.5.2.
2. Ensure that it is fine to require all users of Tarantool releases >= 1.10.8, <= 1.10.15to installtuple-merger` as well. (We already seem to do that in CMake:

   crud/CMakeLists.txt

   Lines 105 to 107 in 2d3d479

   	execute_process(
   	COMMAND bash "-c" "tarantoolctl rocks install tuple-merger 0.0.2"
   	)

   ).
3. Migrate tuple-merger dependency from CMake to rockspec, as well as tuple-keydef one.
4. Remove select_old code and compatibility layers in code. Remove related tests and CI presets. Rework "if there is a merger" condition to strict assertion.
5. Tag a new module major version.
6. File a ticket related to removing tuple-merger dependency in the future: after Tarantool releases >= 1.10.8, <= 1.10.15` support drop there won't be any need for external merger in supported versions.