Make DNS plugin snaps use core20 (#8106)

Fixes #8103.

* Update the DNS plugin generator script to core20 syntax

* Generate new snapcraft.yamls for the DNS plugins

* Update certbot.wrapper to search for python3.8 paths

certbot-dns-dnsimple/snap/snapcraft.yaml

@@ -3,7 +3,7 @@ summary: DNSimple DNS Authenticator plugin for Certbot
 description: DNSimple DNS Authenticator plugin for Certbot
 confinement: strict
 grade: devel
-base: core18
+base: core20
 adopt-info: certbot-dns-dnsimple
 
 parts:
@@ -11,7 +11,6 @@ parts:
     plugin: python
     source: .
     constraints: [$SNAPCRAFT_PART_SRC/snap-constraints.txt]
-    python-version: python3
     override-pull: |
         snapcraftctl pull
         snapcraftctl set-version `grep ^version $SNAPCRAFT_PART_SRC/setup.py | cut -f2 -d= | tr -d "'[:space:]"`
@@ -23,4 +22,4 @@ slots:
     interface: content
     content: certbot-1
     read:
-      - $SNAP/lib/python3.6/site-packages
+      - $SNAP/lib/python3.8/site-packages