Abseil LTS 20210324.0

What's New:

The cleanup library has been released. This library contains the control-flow-construct-like type absl::Cleanup which is used for executing a callback on scope exit.
The numeric library now includes bits.h, a polyfill header containing implementations of C++20's bitwise math functions.
Abseil now installs pkg-config files to make it easier to use Abseil with some other build systems.
Abseil now respects the default CMake installation paths. Standard CMake variables like CMAKE_INSTALL_PREFIX can be used to change the installation path.

The empty absl::container target has been removed from the CMake build. This target had no effect and references to this target in user code can safely be removed.

Baseline: 997aaf3