build: bump minimum required Go version to 1.15

What does this MR do?

Update Go to 1.15.

why ?

1. Go 1.14 is no longer supported (https://golang.org/doc/devel/release)
2. Go 1.17 is expected to be released in August 2021.
3. CNG and Omnibus builder are on 1.16
4. Some dependencies requires modern go versions (labkit is using go 1.15, golangci-lint is using go 1.15)

TODO

• I added the Changelog (e.g. Changelog: feature) trailer to the commits that need to be included in the changelog
• I added unit tests or they are not required
• I added acceptance tests or they are not required
• I added documentation (or it's not required)
• I followed code review guidelines
• I followed Go Style guidelines