gopls/internal/regtest: TestRenamePackage passes with go 1.16
gopls/internal/regtest: TestRenamePackage passes with go 1.16

Instead of skipping the test, insist on 1.16 or later.

See golang/go#41061

Change-Id: Ie13f114039693993d65333ab9194629187a9b6b1
Reviewed-on: https://go-review.googlesource.com/c/tools/+/274533
Run-TryBot: Peter Weinberger <dd140fb9530d791057dd8833ea6768ff65fcde71@google.com>
gopls-CI: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Trust: Peter Weinberger <dd140fb9530d791057dd8833ea6768ff65fcde71@google.com>
Reviewed-by: Rebecca Stambler <d7cf11e5f299d88ea8348366a9fc4f7c335c3afa@golang.org>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools