debug: use dlv-dap as the default debug adapter for local debugging cases

[hyangah]

For local debugging, use dlv-dap as the default debugger.
Remote debugging support still needs work & documentation.

Nightly has been operating in this mode for a while.

• Change the default in the code https://go-review.googlesource.com/c/vscode-go/+/339094
• Update document https://golang.org/cl/339098
• Update welcome page https://golang.org/cl/339099

@suzmue @polinasok

[gopherbot]

Change https://golang.org/cl/339094 mentions this issue: src/goDebugConfiguration: enable dlv-dap for local debugging

[gopherbot]

Change https://golang.org/cl/339099 mentions this issue: src/welcome: mention that dlv-dap is the default debug adapter

[gopherbot]

Change https://golang.org/cl/339098 mentions this issue: docs: move dlv-dap to debugging, debugging to debugging-legacy