chore: update how we handle mod replacements (#6704)

- Add a tidy after replace. The genproto dependency is getting more
  complex as it now associates with many aliases. A tidy is now
  requried.
- Since we are tidying we can no longer just simply drop the replace
  as there would have been dependency updates because of the tidy.
  Instead we can achive the same effect by using git restore.

internal/gapicgen/generator/gapics.go

@@ -245,6 +245,7 @@ func (g *GapicGenerator) addModReplaceGenproto(dir string) error {
 set -ex
 
 go mod edit -replace "google.golang.org/genproto=$GENPROTO_DIR"
+go mod tidy
 `)
 	c.Dir = dir
 	c.Env = []string{
@@ -262,7 +263,8 @@ func (g *GapicGenerator) dropModReplaceGenproto(dir string) error {
 	c := execv.Command("bash", "-c", `
 set -ex
 
-go mod edit -dropreplace "google.golang.org/genproto"
+git restore go.mod
+git restore go.sum 
 `)
 	c.Dir = dir
 	c.Env = []string{