Update build/release toolchain to Go 1.26.6

What would you like to be added: Update the CoreDNS build/release toolchain to Go 1.26.6 while keeping go 1.25.0 as the minimum supported Go version. If appropriate for the project, this could also be expressed with a toolchain go1.26.6 directive in go.mod.

Why is this needed: Go 1.26.6 includes security and bug fixes, including fixes affecting crypto/tls, os, the runtime, compiler, and networking packages. Using the newer toolchain would ensure CoreDNS release binaries benefit from those fixes without unnecessarily raising the project's minimum supported Go version from 1.25.0.