ci(release): remove emqx prebuild OTP25 and ubuntu18.04 (#322)

Author: qzhuyan

.github/workflows/release.yaml

@@ -70,7 +70,6 @@ jobs:
       fail-fast: false
       matrix:
         otp:
-          - 25.3.2-2
           - 26.2.5.2-1
         openssl:
           - openssl3
@@ -83,7 +82,6 @@ jobs:
           - ubuntu24.04
           - ubuntu22.04
           - ubuntu20.04
-          - ubuntu18.04
           - debian12
           - debian11
           - debian10