multiprocessing.eclass: add global-scope ewarn for deprecated < EAPI 7

Signed-off-by: Sam James <sam@gentoo.org>

eclass/multiprocessing.eclass

@@ -1,4 +1,4 @@
-# Copyright 1999-2023 Gentoo Authors
+# Copyright 1999-2024 Gentoo Authors
 # Distributed under the terms of the GNU General Public License v2
 
 # @ECLASS: multiprocessing.eclass
@@ -24,14 +24,18 @@
 # }
 # @CODE
 
-case ${EAPI:-0} in
-	[5678]) ;;
-	*) die "${ECLASS}: EAPI ${EAPI:-0} not supported" ;;
-esac
-
 if [[ -z ${_MULTIPROCESSING_ECLASS} ]]; then
 _MULTIPROCESSING_ECLASS=1
 
+case ${EAPI} in
+	5|6)
+		ewarn "${CATEGORY}/${PF}: ebuild uses ${ECLASS} with deprecated EAPI ${EAPI}!"
+		ewarn "${CATEGORY}/${PF}: Support will be removed on 2024-10-08. Please port to newer EAPI."
+		;;
+	7|8) ;;
+	*) die "${ECLASS}: EAPI ${EAPI:-0} not supported" ;;
+esac
+
 # @FUNCTION: get_nproc
 # @USAGE: [${fallback:-1}]
 # @DESCRIPTION: