Support MarkdownV2 for Telegram

Modern telegram versions support MarkdownV2,
`kapacitor/services/telegram/service.go`
` if parseMode != "" && strings.ToLower(parseMode) != "markdown" && strings.ToLower(parseMode) != "html" {
return "", nil, fmt.Errorf("parseMode %s is not valid, please use 'Markdown' or 'HTML'", parseMode)`
One could add there MarkdownV2

Start in kapacitor/services/telegram/service.go at the parseMode validation shown in the issue. Add MarkdownV2 as an accepted parse mode and update the invalid-value message; done means the service accepts MarkdownV2 while continuing to validate Markdown and HTML.

Written by the indexing model from the issue text.