apk: add OpenWrt 25.12 release build public key

apk: add OpenWrt 25.12 release build public key

Add apk Elliptic Curve public key for signing of package indexes for
upcoming OpenWrt 25.12 release.

apk/openwrt-25.12.pem.gpg is detached GPG signature produced using
following key from Nitrokey 3A Mini dongle:

 pub   ed25519/0x1D53D1877742E911 2023-05-18 [C] [expires: 2033-05-15]
       Key fingerprint = 8A8B C12F 46B8 36C0 F9CD  B36F 1D53 D187 7742 E911
 uid                   [ultimate] OpenWrt Build System (Nitrokey3) <contact@openwrt.org>
 sig 3        0x1D53D1877742E911 2023-05-18  OpenWrt Build System (Nitrokey3) <contact@openwrt.org>
 sig          0xCD84BCED626471F1 2023-05-18  OpenWrt Build System (PGP key for unattended snapshot builds) <pgpsign-snapshots@openwrt.org>
 sig          0xCD54E82DADB3684D 2023-05-18  OpenWrt Build System (GnuPGP key for 22.03 release builds) <pgpsign-22.03@openwrt.org>
 sig          0x88CA59E88F681580 2023-05-18  OpenWrt Build System (PGP key for 21.02 release builds) <pgpsign-21.02@openwrt.org>
 sub   ed25519/0x2B0151090606D1D9 2023-05-18 [S] [expires: 2033-05-15]
       Key fingerprint = 92C5 61DE 55AE 6552 F3C7  36B8 2B01 5109 0606 D1D9
 sig          0x1D53D1877742E911 2023-05-18  OpenWrt Build System (Nitrokey3) <contact@openwrt.org>

Signed-off-by: Petr Štetiar <ynezz@true.cz>