Rename master branch to main (#495)

This updates drone CI to use the new main branch name `main`.

### ⚠️ breaking

The download URLs on https://dl.gitea.io/tea/master will no longer be updated.

@Owners: is there a way to add a redirect for these URLs from `/tea/master` to `/tea/main`?

Co-authored-by: Norwin <git@nroo.de>
Reviewed-on: https://gitea.com/gitea/tea/pulls/495
Co-authored-by: Norwin <noerw@noreply.gitea.io>
Co-committed-by: Norwin <noerw@noreply.gitea.io>

.drone.yml

@@ -43,7 +43,7 @@ steps:
     GOPROXY: https://goproxy.io # proxy.golang.org is blocked in China, this proxy is not
   when:
     branch:
-    - master
+    - main
     event:
     - push
     - pull_request
@@ -156,7 +156,7 @@ steps:
     endpoint: https://ams3.digitaloceanspaces.com
     source: "dist/release/*"
     strip_prefix: dist/release/
-    target: /tea/master
+    target: /tea/main
   environment:
     AWS_ACCESS_KEY_ID:
       from_secret: aws_access_key_id
@@ -164,7 +164,7 @@ steps:
       from_secret: aws_secret_access_key
   when:
     branch:
-    - master
+    - main
     event:
     - push