update references to Go 1.21 in docs (#218)

Signed-off-by: Michael Valdron <mvaldron@redhat.com>

Author: michael-valdron

CONTRIBUTING.md

@@ -16,7 +16,7 @@ contribution. See the [DCO](./DCO) file for details.
 The following are required to work on devfile library:
 
 - Git
-- Go 1.19 or later
+- Go 1.21 or later
 
 ## Code of Conduct
 Before contributing to this repository, see [contributor code of conduct](https://github.com/devfile/api/blob/main/CODE_OF_CONDUCT.md#contributor-covenant-code-of-conduct)

README.md

@@ -2,7 +2,7 @@
 
 <div id="header">
 
-![Go](https://img.shields.io/badge/Go-1.19-blue)
+![Go](https://img.shields.io/badge/Go-1.21-blue)
 [![Apache2.0 License](https://img.shields.io/badge/license-Apache2.0-brightgreen.svg)](LICENSE)
 [![OpenSSF Best Practices](https://www.bestpractices.dev/projects/8231/badge)](https://www.bestpractices.dev/projects/8231)
 [![OpenSSF Scorecard](https://api.securityscorecards.dev/projects/github.com/devfile/library/badge)](https://securityscorecards.dev/viewer/?uri=github.com/devfile/library)