build: update bootstrap req to C++14

We moved to a bootstrap requirement of C++11 in GCC 11, 8 years after
support was stable in GCC 4.8.

It is now 8 years since C++14 was the default mode in GCC 6 (and 9 years
since support was complete in GCC 5), and we have a few bits of optional
C++14 code in the compiler, so it seems a good time to update the bootstrap
requirement again.

The big benefit of the change is the greater constexpr power, but C++14 also
added variable templates, generic lambdas, lambda init-capture, binary
literals, and numeric literal digit separators.

C++14 was feature-complete in GCC 5, and became the default in GCC 6.  5.4.0
bootstraps trunk correctly; trunk stage1 built with 5.3.0 breaks in
eh_data_format_name due to PR69995.

gcc/ChangeLog:

	* doc/install.texi (Prerequisites): Update to C++14.

ChangeLog:

	* configure.ac: Update requirement to C++14.
	* configure: Regenerate.