kerberos: bump minimum to 1.3 (2003-07-08), drop legacy logic

Previous minimum was: 1.2.4 (2002-02-28)

- assume `gssapi/gssapi.h` header for MIT Kerberos.

  Drop logic detecting this header, and drop alternate logic including
  a bare "gssapi.h". Bare `gssapi.h` is Heimdal-specific. MIT Kerberos
  added support for it for Heimdal compatibility on 2006-11-09,
  redirecting to `gssapi/gssapi.h`. MIT Kerberos supported the latter
  header in the 1990s already.

  Ref: 40e1a016f9 (2008-03-06)
  Ref: d119352001 (2006-11-09)

- configure.ac: stop using `HAVE_GSSAPI_GSSAPI_H`.

  Added in 2010 to support "ancient distros such as RHEL-3" where
  `gssapi/gssapi_krb5.h` did not include `gssapi/gssapi.h`.

  MIT Kerberos includes it since commit:
  d9e959edfa (2003-03-06)
  Released in 1.3 (2003-07-08).

  Bump minimum required version to avoid this issue.

  Reverts cca192e58f (2010-04-16)

Ref: https://web.mit.edu/kerberos/dist/historic.html
Ref: https://sources.debian.org/src/krb5/

Closes #18992

configure.ac

@@ -1841,17 +1841,10 @@ if test x"$want_gss" = xyes; then
     ],
     [
       dnl not found, check for MIT
-      AC_CHECK_HEADERS([gssapi/gssapi.h], [], [not_mit=1])
       AC_CHECK_HEADERS(
-        [gssapi/gssapi_generic.h gssapi/gssapi_krb5.h],
+        [gssapi/gssapi.h gssapi/gssapi_generic.h gssapi/gssapi_krb5.h],
         [],
-        [not_mit=1],
-        [
-          AC_INCLUDES_DEFAULT
-          #ifdef HAVE_GSSAPI_GSSAPI_H
-          #include <gssapi/gssapi.h>
-          #endif
-        ])
+        [not_mit=1])
       if test "x$not_mit" = "x1"; then
         dnl MIT not found
         AC_MSG_ERROR([MIT or GNU GSS library required, but not found])