Enable http/2 support (#137)

As per [the documentation](https://pkg.go.dev/net/http#Serve), it doesn't enable HTTP2 by-default, unless we enable it via the `NextProtos` option.

Co-authored-by: Gusted <williamzijl7@hotmail.com>
Reviewed-on: https://codeberg.org/Codeberg/pages-server/pulls/137
Reviewed-by: 6543 <6543@obermui.de>
Co-authored-by: Gusted <gusted@noreply.codeberg.org>
Co-committed-by: Gusted <gusted@noreply.codeberg.org>

server/certificates/certificates.go

@@ -117,6 +117,7 @@ func TLSConfig(mainDomainSuffix string,
 		},
 		PreferServerCipherSuites: true,
 		NextProtos: []string{
+			"h2",
 			"http/1.1",
 			tlsalpn01.ACMETLS1Protocol,
 		},