update funding: added Github Sponsors, removed Flattr

.github/FUNDING.yml

@@ -1,6 +1,6 @@
 # These are supported funding model platforms
 
-github: # Replace with up to 4 GitHub Sponsors-enabled usernames e.g., [user1, user2]
+github: [mtotschnig]
 patreon: # Replace with a single Patreon username e.g., user1
 open_collective: # Replace with a single Open Collective username e.g., user1
 ko_fi: # Replace with a single Ko-fi username e.g., user1
@@ -9,4 +9,4 @@ community_bridge: # Replace with a single Community Bridge project-name e.g., cl
 liberapay: # Replace with a single Liberapay username e.g., user1
 issuehunt: # Replace with a single IssueHunt username e.g., user1
 otechie: # Replace with a single Otechie username e.g., user1
-custom: ['https://www.paypal.com/donate/?hosted_button_id=KPXNZHMXJE8ZJ', 'https://www.myexpenses.mobi/en/#participate', 'https://flattr.com/@michael.totschnig']
+custom: ['https://www.paypal.com/donate/?hosted_button_id=KPXNZHMXJE8ZJ', 'https://www.myexpenses.mobi/en/#participate']