all: require go 1.21+ for tools installation

[hyangah]

The upcoming gopls requires go1.21+ for installation.
golang/go#65917

Update the min version in the README and tools installation logic.

[gopherbot]

Change https://go.dev/cl/588757 mentions this issue: extension/src: require go1.19+ for tools installation

[findleyr]

Putting this in the v0.42.0 milestone. @hyangah please remilestone if that is incorrect.

[gopherbot]

Change https://go.dev/cl/616675 mentions this issue: README.md: clarify supported go & tools versions

[gopherbot]

Change https://go.dev/cl/616676 mentions this issue: extension/src/goInstallTools.ts: require go1.21+ for tools installation