avoid using native-tls in dependencies

[pete-eiger]

As discovered in graphops/subgraph-radio#147 , native-tls is causing memory leak issues in Subgraph Radio. It it used in three of our dependencies - reqwest, teloxide and slack-morphism. reqwest is already dealt with by switching the dependency definition from

reqwest = { version = "0.11.17", features = ["json"] }

to

reqwest = { version = "0.11.24", features = ["json", "rustls-tls"] }

But it might not be so straightforward for the other two. If we can't configure them to use rustls instead of native-tls, we should find alternative crates or methods to support Slack and Telegram bot functionalities.