[tlbuild] icu-76.2 in TL source tree, requires C++17

[Karl Berry]

Hello TL builders - I've installed icu-76.2 in the TL source tree
(libs/icu). It requires C++17 (see "ICU4C Platform Support" near the
bottom of https://unicode-org.github.io/icu/download/76.html).

So I thought I'd send a note here in case people want to adjust their
builds now. There is no escaping it (apart from not building xetex et al.).

For Red Hat-based systems, I was able to complete the build by
installing the package "gcc-toolset-13" and then running the build as
  scl enable gcc-toolset-13 -- ./Build ...other args...

Probably older gcc-toolset-NN packages also provide C++17, but I just
went with the newest available. Amazingly to me, the build of icu and
everything else then went through without incident.

By the way, the only trick I found online to discern which C++ standards 
are supported by a given gcc/g++ is this:
  man gcc | grep -e "-std=c++"

Good luck to us all,
Karl