PlutoPkg: if Pkg.resolve fails, try updating the registries

[fonsp]

When opening a notebook (e.g. from the web), we call Pkg.resolve on the notebook environment. If this fails, we try some strategies one-by-one to automatically fix the problem until it resolves. These strategies are:

1. Force update registries (added in this PR)
2. Remove the Manifest.toml
3. Remove the Manifest.toml and all [compat] entries from the Project.toml (added in Recover gracefully from loading a notebook with non-existing versions #2294)

This should improve the situation of #2280. Before this PR, Pluto would try strategy 2, fail, then strategy 3 which works. But this removes all compat entries for all packages, including packages that had no problems, and installs latest versions for all.
After this PR, strategy 1 would already work, and the notebook would run with the intended package versions.

[github-actions]

Try this Pull Request!

Open Julia and type:

julia> import Pkg
julia> Pkg.activate(temp=true)
julia> Pkg.add(url="https://github.com/fonsp/Pluto.jl", rev="update-registries-if-resolve-fails")
julia> using Pluto