github-ci: remove macOS ver. 10.15

We support current and previous version of macOS.
So let's remove 10.15 from CI.

(cherry picked from commit 28026722)

.github/workflows/osx_10_15.yml

-name: osx_10_15
-
-on:
-  push:
-    branches:
-      - 'master'
-      - '[0-9].[0-9]+'
-      - '**-full-ci'
-    tags:
-      - '**'
-  pull_request:
-    types: [opened, reopened, synchronize, labeled]
-  repository_dispatch:
-    types: [backend_automation]
-  workflow_dispatch:
-
-concurrency:
-  # Update of a developer branch cancels the previously scheduled workflow
-  # run for this branch. However, the 'master' branch, release branch (1.10,
-  # 2.8, etc.), and tag workflow runs are never canceled.
-  #
-  # We use a trick here: define the concurrency group as 'workflow run ID' +
-  # 'workflow run attempt' because it is a unique combination for any run.
-  # So it effectively discards grouping.
-  #
-  # Important: we cannot use `github.sha` as a unique identifier because
-  # pushing a tag may cancel a run that works on a branch push event.
-  group: ${{ (
-    github.ref == 'refs/heads/master' ||
-    github.ref == 'refs/heads/1.10' ||
-    startsWith(github.ref, 'refs/heads/2.') ||
-    startsWith(github.ref, 'refs/tags/')) &&
-    format('{0}-{1}', github.run_id, github.run_attempt) ||
-    format('{0}-{1}', github.workflow, github.ref) }}
-  cancel-in-progress: true
-
-env:
-  CI_MAKE: make -f .travis.mk
-
-jobs:
-  osx_10_15:
-    # Run on pull request only if the 'full-ci' label is set.
-    if: github.event_name != 'pull_request' ||
-        contains(github.event.pull_request.labels.*.name, 'full-ci')
-
-    runs-on: macos-10.15
-
-    steps:
-      - uses: actions/checkout@v2.3.4
-        with:
-          fetch-depth: 0
-          submodules: recursive
-      - uses: ./.github/actions/environment
-      - name: test
-        run: ${CI_MAKE} test_osx_github_actions
-      - 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: osx_10_15
-          retention-days: 21
-          path: ${{ env.VARDIR }}/artifacts