Bump MSRV to 1.56 - 2021 edition

[dralley]

No description provided.

[dralley]

@Mingun As you can see from the failure, even at the present moment, our effective MSRV is higher than 1.52 if you are using various features. I'd rather just bump the baseline then have the MSRV be misleading especially with the other discussion ongoing.

[Mingun]

our effective MSRV is higher than 1.52 if you are using various features.

This is expected. MSRV means absolute minimal version without any optional dependencies. Many our dependents does not use serde functionality, so for them effective MSRV still 1.52. But I made investigation and actually almost all of our dependents use either edition 2021 (this is rust 1.56, right?), or MSRV>=1.56. In some rare cases none of those is specified.

I grabbed versions of some crates (those which set rust-version explicitly, but many other crates specify rust 2021), listed according to https://lib.rs/crates/quick-xml/rev a week ago:

2018=1.31
2021=1.56?

inferno=1.59?
plist=1.59
wayland-scanner=1.65
zbuz=1.64
quick-junit=1.66
phonenumber=1.58.0
opendal=1.65
object_store=1.62.1
calamine=1.63
lcov2cobertura=1.59
tauri-winrt-notification=1.59
poem-openapi=1.64
azure_core=1.70
poem=1.64
gtk4-macros=1.70
starship=1.65
nu-command=1.60
ndk-build=1.60
rio_xml=1.60
wayrs-client=1.65
rusty-s3=1.61
cargo-pants=1.69
mavlink=1.60.0
gvdb=1.65
sparesults=1.60
rpki=1.63
dxr=1.65.0
wiktionary-dump-parser=1.58.1
fefix=1.56
office=1.63
threemf=1.56
calyx=1.66
s3-server=1.59

So I think we can bump our MSRV, but let's test MSRV without optional features. If someone uses serde, it's usually includes it itself and can rely on serde's MSRV directly.