Bump github.com/google/go-cmp from 0.5.6 to 0.5.9

Bumps [github.com/google/go-cmp](https://github.com/google/go-cmp) from 0.5.6 to 0.5.9.
- [Release notes](https://github.com/google/go-cmp/releases)
- [Commits](google/go-cmp@v0.5.6...v0.5.9)

---
updated-dependencies:
- dependency-name: github.com/google/go-cmp
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <support@github.com>

go.mod

@@ -2,6 +2,4 @@ module github.com/transparency-dev/merkle
 
 go 1.17
 
-require github.com/google/go-cmp v0.5.6
-
-require golang.org/x/xerrors v0.0.0-20191204190536-9bdfabe68543 // indirect
+require github.com/google/go-cmp v0.5.9

go.sum

@@ -1,4 +1,2 @@
-github.com/google/go-cmp v0.5.6 h1:BKbKCqvP6I+rmFHt06ZmyQtvB8xAkWdhFyr0ZUNZcxQ=
-github.com/google/go-cmp v0.5.6/go.mod h1:v8dTdLbMG2kIc/vJvl+f65V22dbkXbowE6jgT/gNBxE=
-golang.org/x/xerrors v0.0.0-20191204190536-9bdfabe68543 h1:E7g+9GITq07hpfrRu66IVDexMakfv52eLZ2CXBWiKr4=
-golang.org/x/xerrors v0.0.0-20191204190536-9bdfabe68543/go.mod h1:I/5z698sn9Ka8TeJc9MKroUUfqBBauWjQqLJ2OPfmY0=
+github.com/google/go-cmp v0.5.9 h1:O2Tfq5qg4qc4AmwVlvv0oLiVAGB7enBSJ2x2DqQFi38=
+github.com/google/go-cmp v0.5.9/go.mod h1:17dUlkBOakJ0+DkrSSNjCkIjxS6bF9zb3elmeNGIjoY=