Replace ureq with minreq

[tranzystorekk]

I noticed ureq can be pretty easily replaced with minreq, its minimal-dependency equivalent, which offers a few benefits:

• slightly smaller binary (on x86_64 linux with native-tls size goes down roughly from 2.7MB to 2.4MB)
• minreq handles its tls backend transparently, instead of the custom TlsConnector code

I briefly tested both tls backends by cleaning fend cache and calculating simple currency conversions.

One thing I wasn't sure about: should we use the https-rustls-probe feature to auto-detect native certs? At least on my side I don't see much difference or issues without it.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.

Project coverage is 81.96%. Comparing base (431b6c2) to head (73a42af).

:exclamation: Current head 73a42af differs from pull request most recent head 07c8061

Please upload reports for the commit 07c8061 to get more accurate results.

Files	Patch %	Lines
cli/src/exchange_rates.rs	0.00%	4 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #299 +/- ## ======================================= Coverage 81.96% 81.96% ======================================= Files 52 52 Lines 13831 13830 -1 ======================================= Hits 11336 11336 + Misses 2495 2494 -1 ```

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[printfn]

Thanks for your PR! Yeah, I think enabling https-rustls-probe for detecting native certs makes sense. Just in general, it always seems a bit ridiculous to me that many Rust programs exclusively use their own non-customisable cert store instead of using what's already built into the OS.

If you could add the https-rustls-probe feature I'll go and merge the PR.

[printfn]

Not sure what's going on with that codecov pipeline check, I'll merge this regardless