lib/upgrade: Extract signing key to embedded file (fixes #9247) (#9296)

### Purpose

Instead of hardcoding `SigningKey` as text use `go:embed`. Fixes #9247.

### Testing

* Building syncthing
* Trying to upgrade (signature verification)

lib/upgrade/signingkey.go

@@ -6,14 +6,13 @@
 
 package upgrade
 
+import _ "embed"
+
 // SigningKey is the public key used to verify signed upgrades. It must match
 // the private key used to sign binaries for the built in upgrade mechanism to
 // accept an upgrade. Keys and signatures can be created and verified with the
 // stsigtool utility. The build script creates signed binaries when given the
 // -sign option.
-var SigningKey = []byte(`-----BEGIN EC PUBLIC KEY-----
-MIGbMBAGByqGSM49AgEGBSuBBAAjA4GGAAQA1iRk+p+DsmolixxVKcpEVlMDPOeQ
-1dWthURMqsjxoJuDAe5I98P/A0kXSdBI7avm5hXhX2opJ5TAyBZLHPpDTRoBg4WN
-7jUpeAjtPoVVxvOh37qDeDVcjCgJbbDTPKbjxq/Ae3SHlQMRcoes7lVY1+YJ8dPk
-2oPfjA6jtmo9aVbf/uo=
------END EC PUBLIC KEY-----`)
+//
+//go:embed signingkey.pem
+var SigningKey []byte

lib/upgrade/signingkey.pem

@@ -0,0 +1,6 @@
+-----BEGIN EC PUBLIC KEY-----
+MIGbMBAGByqGSM49AgEGBSuBBAAjA4GGAAQA1iRk+p+DsmolixxVKcpEVlMDPOeQ
+1dWthURMqsjxoJuDAe5I98P/A0kXSdBI7avm5hXhX2opJ5TAyBZLHPpDTRoBg4WN
+7jUpeAjtPoVVxvOh37qDeDVcjCgJbbDTPKbjxq/Ae3SHlQMRcoes7lVY1+YJ8dPk
+2oPfjA6jtmo9aVbf/uo=
+-----END EC PUBLIC KEY-----