Clarify Markdown vs MarkdownV2 support

The [docs](https://github.com/appleboy/telegram-action#input-variables) mention MarkdownV2 support, but based on my own tests it appears that only legacy Markdown syntax is supported.

For example, see the following actions snippet:
```yaml
jobs:
send_message:
runs-on: ubuntu-latest
steps:
- name: Telegram Message Notify
uses: appleboy/telegram-action@v0.1.1
with:
to: ${{ secrets.TELEGRAM_TO }}
token: ${{ secrets.TELEGRAM_BOT_TOKEN }}
message: |
${{ github.actor }} ${{ github.event.action }} PR ${{ github.event.number }}:
[_${{ github.event.pull_request.title }}_](${{ github.event.pull_request.html_url }})
format: markdown
```
Despite not being escaped, the italicized title didn't work as expected:

The [markdown style](https://core.telegram.org/bots/api#markdown-style) notes mention this:
> Entities must not be nested, use parse mode [MarkdownV2](https://core.telegram.org/bots/api#markdownv2-style) instead.

Moreover, [drone-telegram](https://github.com/appleboy/drone-telegram/blob/master/DOCS.md#parameter-reference) seems to indicate `markdown` (v1) is the supported Markdown flavor - not v2.

Can the docs get updated to indicate which flavor of markdown is actually supported?

Start with the input variables documentation linked in the issue and compare its Markdown wording with Telegram's Markdown and MarkdownV2 references and the drone-telegram parameter reference. Confirm which flavor the action supports, then update the documentation to state that flavor clearly and verify the example matches the documented behavior.

Written by the indexing model from the issue text.