Upgrade to the latest version of golang-jwt (#16590) (#16606)

* Upgrade to the latest version of golang-jwt.

Backport #16590

* Forcibly update the vendored versions too
* Update our minimal go lang version to 1.15 (differs from 1.16 in #16590)

Signed-off-by: Andrew Thornton <art27@cantab.net>

 ### ⚠️ BREAKING ⚠️

This PR raises the minimal version of go supported to 1.15 which will mean the end of support of 32-bit Mac and Mac OS versions before Sierra.

Signed-off-by: Andrew Thornton <art27@cantab.net>

* update minimal go required

Signed-off-by: Andrew Thornton <art27@cantab.net>

* update config.yaml

Signed-off-by: Andrew Thornton <art27@cantab.net>

vendor/github.com/golang-jwt/jwt/ed25519_utils.go

@@ -0,0 +1,64 @@
+package jwt
+
+import (
+	"crypto"
+	"crypto/ed25519"
+	"crypto/x509"
+	"encoding/pem"
+	"errors"
+)
+
+var (
+	ErrNotEdPrivateKey = errors.New("Key is not a valid Ed25519 private key")
+	ErrNotEdPublicKey  = errors.New("Key is not a valid Ed25519 public key")
+)
+
+// Parse PEM-encoded Edwards curve private key
+func ParseEdPrivateKeyFromPEM(key []byte) (crypto.PrivateKey, error) {
+	var err error
+
+	// Parse PEM block
+	var block *pem.Block
+	if block, _ = pem.Decode(key); block == nil {
+		return nil, ErrKeyMustBePEMEncoded
+	}
+
+	// Parse the key
+	var parsedKey interface{}
+	if parsedKey, err = x509.ParsePKCS8PrivateKey(block.Bytes); err != nil {
+		return nil, err
+	}
+
+	var pkey ed25519.PrivateKey
+	var ok bool
+	if pkey, ok = parsedKey.(ed25519.PrivateKey); !ok {
+		return nil, ErrNotEdPrivateKey
+	}
+
+	return pkey, nil
+}
+
+// Parse PEM-encoded Edwards curve public key
+func ParseEdPublicKeyFromPEM(key []byte) (crypto.PublicKey, error) {
+	var err error
+
+	// Parse PEM block
+	var block *pem.Block
+	if block, _ = pem.Decode(key); block == nil {
+		return nil, ErrKeyMustBePEMEncoded
+	}
+
+	// Parse the key
+	var parsedKey interface{}
+	if parsedKey, err = x509.ParsePKIXPublicKey(block.Bytes); err != nil {
+		return nil, err
+	}
+
+	var pkey ed25519.PublicKey
+	var ok bool
+	if pkey, ok = parsedKey.(ed25519.PublicKey); !ok {
+		return nil, ErrNotEdPublicKey
+	}
+
+	return pkey, nil
+}