update required go version to build in CONTRIBUTING.md

[fletcm]

Describe the bug
The bug is that CONTRIBUTING.md lists the required version of golang to build the project as 1.14, however make lint fails with this version.

Steps to reproduce
set your local go version to 1.14
make lint fails (output below)
set your local go version to 1.16
make lint now works

What did you expect to see?
Expected it to succeed.

What did you see instead?

$ make lint
golangci-lint run --allow-parallel-runners
WARN [runner] Can't run linter goanalysis_metalinter: S1004: failed prerequisites: [([email protected]/collector/cmd/schemagen/schemagen [go.opentelemetry.io/collector/cmd/schemagen/schemagen.test], [email protected]/collector/cmd/schemagen/schemagen [go.opentelemetry.io/collector/cmd/schemagen/schemagen.test]): analysis skipped: errors in package: [/apps/opentelemetry-collector/cmd/schemagen/schemagen/cli_all_test.go:34:15: t.TempDir undefined (type *testing.T has no field or method TempDir) /apps/opentelemetry-collector/cmd/schemagen/schemagen/cli_single_test.go:75:15: t.TempDir undefined (type *testing.T has no field or method TempDir)]] 
WARN [runner] Can't run linter unused: buildir: analysis skipped: errors in package: [/apps/opentelemetry-collector/cmd/schemagen/schemagen/cli_all_test.go:34:15: t.TempDir undefined (type *testing.T has no field or method TempDir) /apps/opentelemetry-collector/cmd/schemagen/schemagen/cli_single_test.go:75:15: t.TempDir undefined (type *testing.T has no field or method TempDir)] 
ERRO Running error: buildir: analysis skipped: errors in package: [/apps/opentelemetry-collector/cmd/schemagen/schemagen/cli_all_test.go:34:15: t.TempDir undefined (type *testing.T has no field or method TempDir) /apps/opentelemetry-collector/cmd/schemagen/schemagen/cli_single_test.go:75:15: t.TempDir undefined (type *testing.T has no field or method TempDir)] 
make: *** [lint] Error 3

What version did you use?
Version: upstream/main as of yesterday

What config did you use?
Config: (e.g. the yaml config file)

Environment
OS: Mac OSX Mojave 10.14.6

Additional context
The method it is complaining about was introduced in go 1.15. Just need to update CONTRIBUTING.md to reflect this, unless there is a reason to change the code instead to get back compatibility with 1.14.

[bogdandrutu]

I think this is done since I merged that PR. Please reopen if that is not the case