Makefile,*: use tailscale.com/cmd/mkversion

[raggi]

We've suffered misalignment in versioning and toolchain usage due to the shell invocations downstream of ./version/tailscale-version.sh, but also the whole version data scheme in the Makefile was quite complicated, and required synchronization in the build.grade.

• Makfile no longer needs to be version aware itself.
• A Makefile target tailscale.version refreshes a local cached output from tailscale.com/cmd/mkversion which is updated when go.mod / go.sum change.
• build.gradle loads tailscale.version to get the version string.
• ldflags are produced from tailscale.version via version-ldflags.sh

Updates https://github.com/tailscale/tailscale/issues/13850