gopls/internal/test/marker: fix {hover/def}/comment tests

It is currently failing on CI on pre-go1.20 toolchains
due to the too-new go.mod file.

Change-Id: I4cabd40bd50c8bdf084dac113062c04b34d53946
Reviewed-on: https://go-review.googlesource.com/c/tools/+/578038
Reviewed-by: Robert Findley <[email protected]>
Auto-Submit: Alan Donovan <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: adonovan

gopls/internal/test/marker/testdata/definition/comment.txt

@@ -3,7 +3,7 @@ This test executes definition requests over doc links.
 -- go.mod --
 module mod.com
 
-go 1.21
+go 1.19
 
 -- a.go --
 package p

gopls/internal/test/marker/testdata/hover/comment.txt

@@ -8,7 +8,7 @@ Requires go1.20+ for the unsafe package test.
 -- go.mod --
 module mod.com
 
-go 1.21
+go 1.20
 
 -- a.go --
 package p