github-ci: drop Ubuntu Trusty (14.04) support

Reasons, in short:

* It reaches end of the standard support.
* It looks barely usable with old ca-certificates due to Let's Encrypt
  certificate expire problem.
* It has problems that block updating compat level for our *.deb
  packages.

All details and links can be found in the linked issue.

Fixes #6502

.github/workflows/ubuntu_14_04.yml

-name: ubuntu_14_04
-
-on:
-  push:
-  pull_request:
-  repository_dispatch:
-    types: [backend_automation]
-  workflow_dispatch:
-
-jobs:
-  ubuntu_14_04:
-    # We want to run on external PRs, but not on our own internal PRs
-    # as they'll be run by the push to the branch.
-    if: ( github.event_name == 'push' ||
-        github.event.pull_request.head.repo.full_name != github.repository ) &&
-        ! endsWith(github.ref, '-notest')
-
-    runs-on: ubuntu-20.04
-
-    strategy:
-      fail-fast: false
-
-    steps:
-      # Permissions correction is needed for self-host runners,
-      # where work path is saved between different workflows runs.
-      - name: correct permissions in working directory
-        shell: bash
-        run: |
-          sudo chown -R $(id -u):$(id -g) .
-      - uses: actions/checkout@v2.3.4
-        with:
-          fetch-depth: 0
-          submodules: recursive
-      - uses: ./.github/actions/environment
-      - name: packaging
-        env:
-          AWS_ACCESS_KEY_ID: ${{ secrets.AWS_ACCESS_KEY_ID }}
-          AWS_SECRET_ACCESS_KEY: ${{ secrets.AWS_SECRET_ACCESS_KEY }}
-          AWS_S3_ENDPOINT_URL: ${{ secrets.AWS_S3_ENDPOINT_URL }}
-          RELEASE_REPO_S3_DIR: ${{ secrets.RELEASE_REPO_S3_DIR }}
-          PRERELEASE_REPO_S3_DIR: ${{ secrets.PRERELEASE_REPO_S3_DIR }}
-          GPG_SECRET_KEY: ${{ secrets.GPG_SECRET_KEY }}
-          GPG_SIGN_KEY: ${{ secrets.GPG_SIGN_KEY }}
-          OS: 'ubuntu'
-          DIST: 'trusty'
-        uses: ./.github/actions/pack_and_deploy
-      - name: call action to send Telegram message on failure
-        env:
-          TELEGRAM_TOKEN: ${{ secrets.TELEGRAM_CORE_TOKEN }}
-          TELEGRAM_TO: ${{ secrets.TELEGRAM_CORE_TO }}
-        uses: ./.github/actions/send-telegram-notify
-        if: failure()
-      - name: artifacts
-        uses: actions/upload-artifact@v2
-        if: failure()
-        with:
-          name: ubuntu-trusty
-          retention-days: 21
-          path: build/usr/src/*/tarantool-*/test/var/artifacts

changelogs/unreleased/drop-ubuntu-trusty.md

+## feature/build
+
+* Stop support of Ubuntu Trusty (14.04). (gh-6502)