Bump minimum Go version to 1.19

[skip ci]

CHANGELOG.md

@@ -1,3 +1,7 @@
+# unreleased
+
+* Bumped minimum Go version to 1.19.
+
 # v0.8.2 (2023-02-16)
 
 * Updated portal room power levels to always allow poll votes.

go.mod

@@ -1,6 +1,6 @@
 module maunium.net/go/mautrix-whatsapp
 
-go 1.18
+go 1.19
 
 require (
 	github.com/chai2010/webp v1.1.1