Pull request: update snap core version

Merge in DNS/adguard-home from 2306-update-snap-core to master

Closes #2306.

Squashed commit of the following:

commit e02c083
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Mon Nov 23 16:15:59 2020 +0300

    all: update snap core version

.goreleaser.yml

@@ -61,7 +61,7 @@
 
 'snapcrafts':
 - 'name': 'adguard-home'
-  'base': 'core18'
+  'base': 'core20'
   'name_template': '{{ .ProjectName }}_{{ .Arch }}{{ if .Arm }}v{{ .Arm }}{{ end }}'
   'summary': 'Network-wide ads & trackers blocking DNS server'
   'description': |