Update Relx to v3.24.5

I've built and uploaded the newest Relx to https://erlang.mk
Next time this is necessary I should add a Makefile target to
do it automatically.

plugins/relx.mk

@@ -8,7 +8,7 @@
 RELX ?= $(ERLANG_MK_TMP)/relx
 RELX_CONFIG ?= $(CURDIR)/relx.config
 
-RELX_URL ?= https://github.com/erlware/relx/releases/download/v3.23.0/relx
+RELX_URL ?= https://erlang.mk/res/relx-v3.24.5
 RELX_OPTS ?=
 RELX_OUTPUT_DIR ?= _rel
 RELX_REL_EXT ?=