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/symmetrically_encrypted.go

@@ -74,6 +74,10 @@ func (se *SymmetricallyEncrypted) Decrypt(c CipherFunction, key []byte) (io.Read
 // SerializeSymmetricallyEncrypted serializes a symmetrically encrypted packet
 // to w and returns a WriteCloser to which the to-be-encrypted packets can be
 // written.
+// If aeadSupported is set to true, SEIPDv2 is used with the indicated CipherSuite.
+// Otherwise, SEIPDv1 is used with the indicated CipherFunction.
+// Note: aeadSupported MUST match the value passed to SerializeEncryptedKeyAEAD
+// and/or SerializeSymmetricKeyEncryptedAEADReuseKey.
 // If config is nil, sensible defaults will be used.
 func SerializeSymmetricallyEncrypted(w io.Writer, c CipherFunction, aeadSupported bool, cipherSuite CipherSuite, key []byte, config *Config) (Contents io.WriteCloser, err error) {
 	writeCloser := noOpCloser{w}