Downgrade many_single_char_names to pedantic

[dtolnay]

As suggested by @flip1995 in #7666 (comment), by today's standards this lint would be considered pedantic.

This is one of the most widely suppressed Clippy lints on crates.io according to https://github.com/dtolnay/noisy-clippy.

In my opinion this lint is just too domain specific for Clippy to have reliable visibility into. Sure there are some cases where the author is just being lazy and could use a kick in the butt, but we're still left with an enormous number of suppressions where single chars are the most appropriate name. For example in the context of colors, a function using h, s, l, r, g, b is 100% sensible and spelling all those out is silly, but it's past the default lint threshold.

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changelog: Moved [many_single_char_names] to pedantic

[rust-highfive]

r? @xFrednet

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[xFrednet]

The changes LGTM. Thank you for the updated list in #7666 and this PR!

Note: I've updated the change log entry to state the group the lint moved to 🙃

@bors r+

[bors]

📌 Commit c2783c1 has been approved by xFrednet

[bors]

⌛ Testing commit c2783c1 with merge 2316f4d...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: xFrednet
Pushing 2316f4d to master...