Add Go 1.20 Compatibility

[roger2hk]

The cloud build starts using Go 1.20. The changes below are necessary to support Go 1.20.

• Fix deprecated go test -i flag
• Bump golangci-lint from v1.50.1 to v1.51.1
• Fix loopclosure: loop variable test captured by func literal
• Remove deprecated global random number generator rand.Seed in trillian/integration

Checklist

• I have updated the CHANGELOG.
  • Adjust the draft version number according to semantic versioning rules.
• I have updated documentation accordingly.

[roger2hk]

@AlCutter There are staticcheck warning messages for rand.Seed(*seed) as Go 1.20 now automatically seeds the global random number generator (used by top-level functions like Float64 and Int) with a random value. There is no way to get the automatically generated seed. It is recommended to use rand.New(rand.NewSource(seed)) which returns the a new Rand.

Should we temporarily create the golangci-lint exclude rule with SA1019 and these two files? There is no way to retrieve the automatically generated seed in Go 1.20. If we use the recommended rand.New(rand.NewSource(seed)), the newly created Rand needs to be passed into all rand functions in other packages. This Go playground example shows the difference.

trillian/integration/ct_hammer/main.go:184:2: SA1019: rand.Seed has been deprecated since Go 1.20 and an alternative has been available since Go 1.0: Programs that call Seed and then expect a specific sequence of results from the global random source (using functions such as Int) can be broken when a dependency changes how much it consumes from the global random source. To avoid such breakages, programs that need a specific result sequence should use NewRand(NewSource(seed)) to obtain a random generator that other packages cannot access. (staticcheck)
	rand.Seed(*seed)
	^
trillian/integration/ct_integration_test.go:56:2: SA1019: rand.Seed has been deprecated since Go 1.20 and an alternative has been available since Go 1.0: Programs that call Seed and then expect a specific sequence of results from the global random source (using functions such as Int) can be broken when a dependency changes how much it consumes from the global random source. To avoid such breakages, programs that need a specific result sequence should use NewRand(NewSource(seed)) to obtain a random generator that other packages cannot access. (staticcheck)
	rand.Seed(*seed)
	^

The [math/rand](https://tip.golang.org/pkg/math/rand/) package now automatically seeds the global random number generator (used by top-level functions like Float64 and Int) with a random value, and the top-level [Seed](https://tip.golang.org/pkg/math/rand/#Seed) function has been deprecated. Programs that need a reproducible sequence of random numbers should prefer to allocate their own random source, using rand.New(rand.NewSource(seed)).

Programs that need the earlier consistent global seeding behavior can set GODEBUG=randautoseed=0 in their environment.