internal/goversion: update Version to 1.19

This is the start of the Go 1.19 development cycle, so update the
Version value accordingly. It represents the Go 1.x version that will
soon open up for development (and eventually become released).

Updates #40705
Updates #51336

Change-Id: Ic4b3f2c04b1fa5c588cb6d62e829f2ed1864e511
Reviewed-on: https://go-review.googlesource.com/c/go/+/388376
Trust: Carlos Amedee <[email protected]>
Run-TryBot: Carlos Amedee <[email protected]>
Trust: Alex Rakoczy <[email protected]>
Reviewed-by: Bryan Mills <[email protected]>
Reviewed-by: Alex Rakoczy <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>

Author: cagedmantis

src/cmd/go/testdata/script/work_edit.txt

@@ -159,4 +159,4 @@ use (
 replace (
 	x.1 v1.3.0 => y.1 v1.4.0
 	x.1 v1.4.0 => ../z
-)
+)

src/internal/goversion/goversion.go

@@ -9,4 +9,4 @@ package goversion
 //
 // It should be updated at the start of each development cycle to be
 // the version of the next Go 1.x release. See golang.org/issue/40705.
-const Version = 18
+const Version = 19