Upgrade zeroconf to 0.21.2

[fabaff]

Description:

Changelog: https://github.com/jstasiak/python-zeroconf/#0211

Example entry for configuration.yaml (if applicable):

zeroconf:

$ avahi-browse -alr
[...]
$ avahi-browse -alr
= enp0s20f0u6u1 IPv4 Home                                          _home-assistant._tcp local
   hostname = [Home._home-assistant._tcp.local]
   address = [192.168.0.241]
   port = [8123]
   txt = ["requires_api_password=false" "base_url=http://192.168.0.241:8123" "version=0.79.0.dev0"]

Checklist:

• The code change is tested and works locally.
• Local tests pass with tox. Your PR cannot be merged unless tests pass

If the code communicates with devices, web services, or third-party tools:

• New dependencies have been added to the REQUIREMENTS variable (example).
• New dependencies have been added to requirements_all.txt by running script/gen_requirements_all.py.