[note]: Can't move abseil-cpp beyond 20240116.* until bazel 6 is retired

[hzeller]

This is mostly to document and keep track of the unfortunate fact that we're stuck on an older abseil-cpp 20240116.2, as newer absl won't compile with older versions of bazel: it only works starting from bazel 7 as they switched from googletest 1.14.0 to depend on googletest 1.15.0 that requires at least bazel 7.

Since we want to be compatible with all maintained bazel versions, we want to support Bazel 6 ... bazel 8. According to https://bazel.build/release this means we're stuck with the last version of abseil-cpp that supports bazel 6 till Dec 2025 (also on googletest 1.14.*).

This is documented in our MODULE.bazel:

verible/MODULE.bazel

Lines 18 to 23 in 3742532

	# abseil-cpp and googletest can not be updated beyond the following currently,
	# as newer googletest is not compatible with bazel 6 anymore, and abseil-cpp
	# depends on it :( -- to support all active bazel's, we're stuck till EOL bazel6
	bazel_dep(name = "googletest", version = "1.14.0.bcr.1", dev_dependency = True)
	bazel_dep(name = "abseil-cpp", version = "20240116.2")

There is one caveat: we must assume that someone might use Verible as dependency that uses a newer version of abseil-cpp. So we also need to be compatible with later abseil-cpp even though our primary one is the older versions; entering the realm of #ifdef :)

We have shim-code that makes Verible compile with old and new abseil-cpp; most notably replacing a deprecated function call and including vlog_is_on.h as that became part of the public interface.

[ No action needed for users: anyone using verible should be able to compile with newer abseil-cpp. ]

This issue is to record all the things that needed to be done for the old/new compatibility, and serves as reference what to clean up once we are beyond December 2025 when we finally can switch to newer absl.