doc: Update install.texi for GCC 13

doc: Update install.texi for GCC 13

install.texi needs some updates for GCC 13 and trunk:

* We used a mixture of Solaris 2 and Solaris references.  Since Solaris
  1/SunOS 4 is ancient history by now, consistently use Solaris
  everywhere.  Likewise, explicit references to Solaris 11 can go in
  many places since Solaris 11.3 and 11.4 is all GCC supports.

* Some caveats apply to both Solaris/SPARC and x86, like the difference
  between as and gas.

* Some specifics are obsolete, like the /usr/ccs/bin path whose contents
  was merged into /usr/bin in Solaris 11.0 already.  Likewise, /bin/sh
  is ksh93 since Solaris 11.0, so there's no need to explicitly use
  /bin/ksh.

* I've removed the reference to OpenCSW: there's barely a need for external
  sites to get additional packages.  OpenCSW is mostly unmaintained these
  days and has been found to be rather harmful then helping.

* The section on assembler and linker to use was partially duplicated.
  Better keep the info in one place.

* GNAT is bundled in recent Solaris 11.4 updates, so recommend that.

Tested on i386-pc-solaris2.11 with make doc/gccinstall.{info,pdf} and
inspection of the latter.

2023-04-21  Rainer Orth  <ro@CeBiTec.Uni-Bielefeld.DE>

	gcc:
	* doc/install.texi: Consistently use Solaris rather than Solaris 2.
	Remove explicit Solaris 11 references.
	Markup fixes.
	(Options specification, --with-gnu-as): as and gas always differ
	on Solaris.
	Remove /usr/ccs/bin reference.
	(Installing GCC: Binaries, Solaris (SPARC, Intel)): Remove.
	(i?86-*-solaris2*): Merge assembler, linker recommendations ...
	(*-*-solaris2*): ... here.
	Update bundled GCC versions.
	Don't refer to pre-built binaries.
	Remove /bin/sh warning.
	Update assembler, linker recommendations.
	Document GNAT bootstrap compiler.
	(sparc-sun-solaris2*): Remove non-UltraSPARC reference.
	(sparc64-*-solaris2*): Move content...
	(sparcv9-*-solaris2*): ...here.
	Add GDC for 64-bit bootstrap compilers.