Fortran manual: Combine standard conformance docs in one place.

Fortran manual: Combine standard conformance docs in one place.

Discussion of conformance with various revisions of the
Fortran standard was split between two separate parts of the
manual.  This patch moves it all to the introductory chapter.

2021-11-01  Sandra Loosemore  <sandra@codesourcery.com>

	gcc/fortran/
	* gfortran.texi (Standards): Move discussion of specific
	standard versions here....
	(Fortran standards status): ...from here, and delete this node.