Discourage use of broken hashes like SHA1

## Problem

> Suggestion extracted from https://github.com/ipfs/go-ipfs/issues/8650

[SHA1 is considered to be broken](https://en.wikipedia.org/wiki/SHA-1#Attacks), and systems like git moved away from it.

IPFS makes migration wa easy thanks to abstraction provided by Multihash. go-ipfs switched the defaults in `ipfs add` to SHA2-256, but in theory, one can still use SHA1 to add new files if they opt-in.

We should have a mechanism to discourage users from using broken legacy functions like sha1 for adding new files.

## Solution

There should be a list of "legacy/deprecated" hash functions like SHA1 which would be discouraged by requiring explicit `--force` flag:

```console
$ ipfs add --hash=sha1 file
Error: selected hash function is no longer secure; use --hash=sha2-256 or pass --force

$ ipfs add --hash=sha1 file
Added ...
```

Note: this could be a part of a bigger refactor related to https://github.com/ipfs/go-ipfs/issues/4371