x/build/internal/task: update the minimum macOS version in the Go 1.21 installer

[dmitshur]

For Go 1.21, per accepted proposal #57125, we should update the macOS minimum version requirement from 10.13 to 10.15 in the installer. (The Go 1.20 installer need not change.)

The mapping from Go releases to minimum macOS version may fit best in the releasetargets package.

(This is similar to #58028.)

CC @golang/release.

[gopherbot]

Change https://go.dev/cl/478617 mentions this issue: internal/releasetargets: formalize minimum macOS version, bump for 1.21