KTailctl v0.21.4

No user-facing changes, only internal dependency updates.

Chores

Flatpak:

• Update org.kde.Sdk from 24.08 to 25.08
• Update org.freedesktop.Sdk.Extension.golang from 24.08 to 25.08

Go:

• Update golang.org/x/net from v0.46.0 to v0.47.0
• Update golang.org/x/crypto from v0.43.0 to v0.45.0
• Update tailscale.com from v1.88.3 to v1.90.9
• Update go toolchain for flatpak: 24.08 (go 1.25.1) -> 25.08 (go 1.25.4)

GitHub Actions:

• Update actions/checkout from v5 to v6