github-ci: send Telegram notifications on failures

Used Telegram public channel to send notifications for failed Github
Actions workflows. Created new Github Action based on Python:

  .github/actions/send-telegram-notify/action.yml

with the ability to send messages in MarkdownV2 format [1]. To make
able to run standalone Github Action used its 'composite' type [2].
Also added ability to logout Github environment variables in this
action.

[1] - https://core.telegram.org/bots/api#markdownv2-style
[2] - https://docs.github.com/en/actions/creating-actions/creating-a-composite-run-steps-action