Update to go 1.15.7

[raskchanky]

This doesn't contain the SDK update, because it looked like from the docs that should only happen on a release branch.

[ncabatoff]

Other places that reference 1.15.4: @executors.yml, go_test.yml, scripts/docker/Dockerfile. These won't impact the release but they should be kept in sync.

[mladlow]

Could you add a changelog file like:

```release-note:change
go: Update go version to 1.15.7
```

? I'm still smoothing out some of the rough edges of this process.

[mladlow]

Thanks!