Revert "package/{protobuf, python-protobuf}: bump to version 3.17"

This reverts commit 92332d31d590f731a55926166dd2da8181c8fcaf, which was
incorrectly applied to master instead of next.

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

package/protobuf/protobuf.hash

@@ -1,3 +1,3 @@
 # Locally calculated
 sha256  6e5e117324afd944dcf67f36cf329843bc1a92229a8cd9bb573d7a83130fea7d  LICENSE
-sha256  84d30a78cae000be164ac6b973ee1dc0664669d63685dfbcf96f058c50456f19  protobuf-cpp-3.17.0.tar.gz
+sha256  bbdfb7455431d7d58666e8a2996d14b236718ff238eecde10646581e4c87f168  protobuf-cpp-3.15.6.tar.gz

package/protobuf/protobuf.mk

@@ -7,7 +7,7 @@
 # When bumping this package, make sure to also verify if the
 # python-protobuf package still works and to update its hash,
 # as they share the same version/site variables.
-PROTOBUF_VERSION = 3.17.0
+PROTOBUF_VERSION = 3.15.6
 PROTOBUF_SOURCE = protobuf-cpp-$(PROTOBUF_VERSION).tar.gz
 PROTOBUF_SITE = https://github.com/google/protobuf/releases/download/v$(PROTOBUF_VERSION)
 PROTOBUF_LICENSE = BSD-3-Clause

package/python-protobuf/python-protobuf.hash

@@ -1,3 +1,3 @@
 # Locally calculated
-sha256  3011bfbc18d1c695ee393cb0dc1042282cfa33ef6573fe82732aefc9f7286121  protobuf-python-3.17.0.tar.gz
+sha256  7e8c58a14950ac7a727864d5c521f48c81feb66ff7df325487fc930c9aa50d8b  protobuf-python-3.15.6.tar.gz
 sha256  6e5e117324afd944dcf67f36cf329843bc1a92229a8cd9bb573d7a83130fea7d  LICENSE