feat(dist): add notes for stable and beta in components_missing_msg()

[rami3l]

Prepares for rls' removal in the release channel.

Continuation of #3453, as requested by @onur-ozkan in rust-lang/rust#126856 (comment).

The best thing we could do is to add a note saying some components may no longer be available (this note currently doesn't apply for stables), but:

1. We'll have to wait for v1.28 to ship it, and we're currently in the middle of the release cycle (we have limited bandwidth so we decided not to do backports), and
2. We should not hardcode the list of removed components in Rustup

https://rust-lang.zulipchat.com/#narrow/stream/241545-t-release/topic/excluding.20rls.20from.20the.20release/near/447968780

The error message is not final, since I'd still like to add a link that provides more info about removed components (which is rls this time): The message now includes a link to https://rust-lang.github.io/rustup/devel/concepts/components.html#previous-components.

... and for this I suggest adding a section in the Rust docs somewhere, and our notes should advise the user to check out the section above to see whether the component in question is missing as intended from what date (and if that is the case, they can choose to unblock the update with a clear consent by removing the component).

But in any way we cannot simply skip RLS in an update if it's previously installed, since that would be a real backwards compatibility problem.
https://rust-lang.zulipchat.com/#narrow/stream/241545-t-release/topic/excluding.20rls.20from.20the.20release/near/447968780

Concerns

• To which document should we add the list of removed components? To the Rustup User Guide, see above for details.
• Should the note be applied to nightly as well? No, as feat(html): add a link to removed components rustup-components-history#54 will add a link to https://rust-lang.github.io/rustup/devel/concepts/components.html#previous-components on the components history page.

[rami3l]

• Should the note be applied to nightly as well?

@onur-ozkan My apologies for getting a bit sidetracked previously! I would like to know how https://rust-lang.github.io/rustup-components-history will behave on the removal of rls, since we include this link in the message for nightly users.

IMHO, it should also include a link to https://rust-lang.github.io/rustup/devel/concepts/components.html#previous-components in case someone wonders why rls is gone in consecutive nightlies starting from a certain date.

[onur-ozkan]

@onur-ozkan My apologies for getting a bit sidetracked previously! I would like to know how https://rust-lang.github.io/rustup-components-history will behave on the removal of rls, since we include this link in the message for nightly users.

I couldn't find any hard-coded logic in https://github.com/rust-lang/rustup-components-history. I think it will just disappear without causing any issues.

IMHO, it should also include a link to https://rust-lang.github.io/rustup/devel/concepts/components.html#previous-components in case someone wonders why rls is gone in consecutive nightlies starting from a certain date.

That sounds great but I don't know who maintains it.

[rami3l]

IMHO, it should also include a link to https://rust-lang.github.io/rustup/devel/concepts/components.html#previous-components in case someone wonders why rls is gone in consecutive nightlies starting from a certain date.

That sounds great but I don't know who maintains it.

cc @Mark-Simulacrum@mexus

[Mark-Simulacrum]

I'm happy to merge small PRs to rustup-components-history.