cmd/go/internal/bug: sync issue body format with GitHub template

[dmitshur]

I've made a proposal to modify the GitHub issue template in #27914. It was accepted and recently implemented in CL 138295. /cc @andybons

From what I've seen so far in the last few days, it has lead to better formatting of issue reports and increased their readability.

If others are equally happy with it, we should backport it to the format that the go bug command uses, so they're in sync:

go/src/cmd/go/internal/bug/bug.go

Lines 72 to 86 in 6fe8ee7

	const bugHeader = `Please answer these questions before submitting your issue. Thanks!
	#### What did you do?
	If possible, provide a recipe for reproducing the error.
	A complete runnable program is good.
	A link on play.golang.org is best.
	#### What did you expect to see?
	#### What did you see instead?
	`

go/.github/ISSUE_TEMPLATE

Lines 3 to 36 in 6fe8ee7

	### What version of Go are you using (`go version`)?
	<pre>
	$ go version
	</pre>
	### Does this issue reproduce with the latest release?
	### What operating system and processor architecture are you using (`go env`)?
	<details><summary><code>go env</code> Output</summary><br><pre>
	$ go env
	</pre></details>
	### What did you do?
	<!--
	If possible, provide a recipe for reproducing the error.
	A complete runnable program is good.
	A link on play.golang.org is best.
	-->
	### What did you expect to see?
	### What did you see instead?

There's no rush on this, as the implementation is trivial, as long as the decision is made before 1.12.

[myitcv]

@dmitshur FYI https://go-review.googlesource.com/c/go/+/127495

@joshblakeley - probably a good time to finish up your changes in that CL :)

[dmitshur]

Thanks Paul. I think I can close this issue as duplicate of #26751 then.