Use caddy's certmagic library for extensible/robust ACME handling (#14177)

* use certmagic for more extensible/robust ACME cert handling

* accept TOS based on config option

Signed-off-by: Andrew Thornton <art27@cantab.net>

Co-authored-by: zeripath <art27@cantab.net>
Co-authored-by: Lauris BH <lauris@nix.lv>

docs/config.yaml

@@ -19,7 +19,7 @@ params:
   author: The Gitea Authors
   website: https://docs.gitea.io
   version: 1.13.1
-  minGoVersion: 1.13
+  minGoVersion: 1.14
   goVersion: 1.15
   minNodeVersion: 10.13