[tests] remove redundant 1.2 certification tests and CI job (#12883)

This commit removes the following legacy 1.2 certification test scripts:
- tests/scripts/thread-cert/v1_2_router_5_1_1.py
- tests/scripts/thread-cert/v1_2_test_parent_selection.py

It also removes the 'packet-verification-1-1-on-1-4' job from the
Simulation 1.4 workflow as it is no longer required.

.github/workflows/simulation-1.4.yml

@@ -121,59 +121,6 @@ jobs:
         path: tmp/coverage.info
         retention-days: 1
 
-  packet-verification-1-1-on-1-4:
-    runs-on: ubuntu-24.04
-    env:
-      REFERENCE_DEVICE: 1
-      VIRTUAL_TIME: 1
-      OT_VT_USE_UNIX_SOCKET: 1
-      PACKET_VERIFICATION: 1
-      THREAD_VERSION: 1.4
-      INTER_OP_BBR: 1
-      MULTIPLY: 3
-    steps:
-    - name: Harden Runner
-      uses: step-security/harden-runner@5ef0c079ce82195b2a36a210272d6b661572d83e # v2.14.2
-      with:
-        egress-policy: audit # TODO: change to 'egress-policy: block' after couple of runs
-
-    - uses: actions/checkout@08c6903cd8c0fde910a37f88322edcfb5dd907a8 # v5.0.0
-      with:
-        submodules: recursive
-    - uses: actions/setup-python@a309ff8b426b58ec0e2a45f0f869d46889d02405 # v6.2.0
-      with:
-        python-version: '3.12'
-        cache: pip
-    - name: Bootstrap
-      run: |
-        sudo apt-get update
-        sudo apt-get --no-install-recommends install -y ninja-build lcov
-        python3 -m pip install -r tests/scripts/thread-cert/requirements.txt
-    - name: Build
-      run: |
-        ./script/test build
-    - name: Get Thread-Wireshark
-      run: |
-        ./script/test get_thread_wireshark
-    - name: Run
-      run: |
-        ./script/test cert_suite ./tests/scripts/thread-cert/Cert_*.py ./tests/scripts/thread-cert/test_*.py
-    - uses: actions/upload-artifact@bbbca2ddaa5d8feaa63e36b76fdaad77386f024f # v7.0.0
-      if: ${{ failure() }}
-      with:
-        name: packet-verification-1.1-on-1.4-pcaps
-        path: |
-          *.pcap
-          *.json
-    - name: Generate Coverage
-      run: |
-        ./script/test generate_coverage gcc
-    - uses: actions/upload-artifact@bbbca2ddaa5d8feaa63e36b76fdaad77386f024f # v7.0.0
-      with:
-        name: cov-packet-verification-1-1-on-1-4
-        path: tmp/coverage.info
-        retention-days: 1
-
   channel-manager-csl:
     runs-on: ubuntu-24.04
     env:
@@ -276,7 +223,6 @@ jobs:
   upload-coverage:
     needs:
     - thread-1-4
-    - packet-verification-1-1-on-1-4
     - expects
     runs-on: ubuntu-22.04
     steps: