Intro: emphasis on year-named editions can make the book seem outdated

[jruderman]

• I have checked the latest main branch to see if this has already been fixed
• I have searched existing issues and pull requests for duplicates

URL to the section(s) of the book with this problem:

https://doc.rust-lang.org/book/

Description of the problem:

The emphasis on "2018 edition" on the first page made me think the book was outdated, because I didn't know about the distinction between Rust editions and Rust versions when I started reading.

This version of the text assumes you’re using Rust 1.54 or later with edition="2018" in Cargo.toml of all projects to use Rust 2018 Edition idioms.

Suggested fix:

• Add "(2021 July)" next to "Rust 1.54"
• Replace "to use Rust 2018 idioms" with "to use its newest edition-gated features"
• Add a brief explanation of editions here and how they differ from versions
• Indicate that "the 2018 edition" is current as of early 2021
• Or, move this whole page to the 'Editions' appendix, and open the book with what is currently the Foreward

[jruderman]

May be related to discussion in #2824