contrib: update nettle URL

The previous URL was not working, so we updated it to the new one:
https://ftp.gnu.org/gnu/nettle/nettle-3.9.1.tar.gz

Change-Id: I1dd4f8ebb918e3ce1b7c9e2fa62010b25861b59d

contrib/src/nettle/SHA512SUMS

-4938d31d9183fd143c6d43a43bda8372bd8899c51c18dfb8640065bffd4c5928006481d16679b1ad057404697f7af06bf1630de4b8072592c38cbb7113d16b21  nettle-nettle_3.9.1_release_20230601.tar.gz
+5939c4b43cf9ff6c6272245b85f123c81f8f4e37089fa4f39a00a570016d837f6e706a33226e4bbfc531b02a55b2756ff312461225ed88de338a73069e031ced  nettle-3.9.1.tar.gz

contrib/src/nettle/rules.mak

 # Nettle
 
-NETTLE_VERSION := nettle_3.9.1_release_20230601
+NETTLE_VERSION := 3.9.1
 PKG_CPE += cpe:2.3:a:nettle_project:nettle:3.9.1:*:*:*:*:*:*:*
-NETTLE_URL := https://git.lysator.liu.se/nettle/nettle/-/archive/$(NETTLE_VERSION)/nettle-$(NETTLE_VERSION).tar.gz
+NETTLE_URL := https://ftp.gnu.org/gnu/nettle/nettle-$(NETTLE_VERSION).tar.gz
 PKGS += nettle
 
 ifeq ($(call need_pkg,"nettle >= 3.6"),)