Remove obsolete compiler version checks from storage/rocksdb/CMakeLists.txt

This is a subtask to account for an incoming contribution to remove the obsolete checks in storage/rocksDB.

Part 1 of MDEV-35462, removing obsolete compiler version checks.

Changes

Simplified storage/rocksdb/CMakeLists.txt by removing:

GCC < 4.8 and < 5.0 version checks
Clang < 3.3 version checks
CMake < 3.0 fallback logic
Manual -std=c++11 flag handling
Replaced with modern CMAKE_CXX_STANDARD configuration.

Confirmed Minimums

As confirmed by @marko:

GCC >= 7
Clang >= 10
CMake >= 3.12
Use CMAKE_CXX_STANDARD for C++11 configuration.

Related

JIRA: https://jira.mariadb.org/browse/MDEV-35462
Discussion: https://mariadb.zulipchat.com/#narrow/channel/118760-New-Members/topic/MDEV-35462.20-.20Compiler.20Cleanup
No functional changes intended.