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/opaque.go

@@ -7,7 +7,6 @@ package packet
 import (
 	"bytes"
 	"io"
-	"io/ioutil"
 
 	"github.com/ProtonMail/go-crypto/openpgp/errors"
 )
@@ -26,7 +25,7 @@ type OpaquePacket struct {
 }
 
 func (op *OpaquePacket) parse(r io.Reader) (err error) {
-	op.Contents, err = ioutil.ReadAll(r)
+	op.Contents, err = io.ReadAll(r)
 	return
 }