devserver: Deprecate setnexupdate API

This API is not used anywhere, deprecate it.

Also deprecates forced_update_label attribute.

BUG=chromium:872441
TEST=devserver_integration_test.py
TEST=autoupdate_unittest.py
TEST=./devserver.py

Change-Id: I243a83e9a0f37884eb1092ee44b2da1cf2cedddb
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/dev-util/+/1828165
Commit-Queue: Amin Hassani <ahassani@chromium.org>
Tested-by: Amin Hassani <ahassani@chromium.org>
Reviewed-by: David Haddock <dhaddock@chromium.org>
4 files changed