Require C++11 in Boost 1.81

[Flamefire]

It might be worth dropping support for C++98. In some PRs move-support was requested and some code places may benefit from defaulted functions.
So C++11 features I'd like to use:

• Rvalue references (move-assign/construct where useful)
• std::unique_ptr, dropping std::auto_ptr which removes a ton of warnings
• defaulted functions

Nothing really big but keeping support for C++98 makes the code harder to write and read due to ifdef-macros etc.
Additionally ICU now requires C++11 as they use char16_t etc. in some of their headers. So building Boost.Locale against such an ICU version requires C++11 anyway.

[artyom-beilis]

Back than it was important to keep C++98/03 support, however today, 11 years after C++11 was released I think dropping C++03 is Ok, especially if it simplifies maintenance.

Finally today, if keeping C++03 support becomes harder than just drop it.

Additionally ICU now requires C++11 as they use char16_t

Irony. I wish they did something in C++ ish way (and not Java written in C++)... Now they require C++11.