fix(cmake): use case-insensitive CMAKE_BUILD_TYPE comparisons

[ddinu]

Description

The $<CONFIG:tgt> generator expression is case-insensitive, this change aligns the behavior of the explicit CMAKE_BUILD_TYPE comparisons used to determine whether debug symbols should be stripped.

I ran into this problem with a codebase that uses DEBUG instead of Debug for CMAKE_BUILD_TYPE, so debug symbols get stripped for debug builds.

Suggested changelog entry:

Avoid stripping debug symbols when `CMAKE_BUILD_TYPE` is set to `DEBUG` instead of `Debug`