chore: modernize CI and update Go toolchain

- Bump Go from 1.19 to 1.26 and update all dependencies
- Rewrite CI workflow with matrix strategy (Linux, macOS, Windows)
- Update GitHub Actions to current versions (checkout@v4, setup-go@v5)
- Update CodeQL actions from v1 to v3
- Fix cross-platform bug in mock/path.go (path.Join -> filepath.Join)
- Clean up dependabot config (weekly schedule, remove stale ignore)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

vendor/github.com/ProtonMail/go-crypto/openpgp/packet/userattribute.go

@@ -9,7 +9,6 @@ import (
 	"image"
 	"image/jpeg"
 	"io"
-	"io/ioutil"
 )
 
 const UserAttrImageSubpacket = 1
@@ -63,7 +62,7 @@ func NewUserAttribute(contents ...*OpaqueSubpacket) *UserAttribute {
 
 func (uat *UserAttribute) parse(r io.Reader) (err error) {
 	// RFC 4880, section 5.13
-	b, err := ioutil.ReadAll(r)
+	b, err := io.ReadAll(r)
 	if err != nil {
 		return
 	}