go.mod: upgrade to 1.20

Upgrade the minimum Go version for the project. This update is necessary to
take advantage of some useful new features (including, in an upcoming patch,
the ability to convert between variable- and fixed-length byte slices).

Signed-off-by: Victoria Dye <vdye@github.com>

Author: vdye

go.mod

@@ -1,6 +1,6 @@
 module github.com/git-ecosystem/git-bundle-server
 
-go 1.19
+go 1.20
 
 require (
 	github.com/google/uuid v1.3.0