Stop pinning dependencies

[dhardy]

Motivation

As a library, rand should be compatible with all dependencies automatically found by the resolver, not only pinned versions.

Details

CI will now test against three version sets:

• The MSRV test (Ubuntu, Rust 1.85.0) will find dependencies using the MSRV-aware resolver. This will include updates to dependencies where they do not require a more recent version of Rust.
• The minimal-versions test (Ubuntu, nightly Rust) will test the minimum version set matching the dependency specification
• Other tests (multiple platforms, stable/beta/nightly Rust) will use the latest versions matching the dependency specification (without MSRV-aware resolution).

We will no longer get explicit tests on new versions via dependabot but we will still get weekly CI re-runs using the latest versions.

Note: this reverts #1699 aside from the part already reverted by #1745.