Downgrade let_unit_value to pedantic

Given that the false positive in rust-lang#1502 is marked E-hard and I don't have much hope of it getting fixed, I think it would be wise to disable this lint by default. I have had to suppress this lint in every substantial codebase (\>100k line) I have worked in. Any time this lint is being triggered, it's always the false positive case.

The motivation for this lint is documented as:

> A unit value cannot usefully be used anywhere. So binding one is kind of pointless.

with this example:

> ```rust
> let x = {
>     1;
> };
> ```

Sure, but the author would find this out via an unused_variable warning or from `x` not being the type that they need further down. If there ends up being a type error on `x`, clippy's advice isn't going to help get the code compiling because it can only run if the code already compiles.

changelog: Remove let_unit_value from default set of enabled lints

Rollup of 12 pull requests

Successful merges:

 - #5345 (Add lint for float in array comparison)
 - #5406 (Fix update_lints)
 - #5409 (Downgrade let_unit_value to pedantic)
 - #5410 (Downgrade trivially_copy_pass_by_ref to pedantic)
 - #5412 (Downgrade inefficient_to_string to pedantic)
 - #5415 (Add new lint for `Result<T, E>.map_or(None, Some(T))`)
 - #5417 (Update doc links and mentioned names in docs)
 - #5419 (Downgrade unreadable_literal to pedantic)
 - #5420 (Downgrade new_ret_no_self to pedantic)
 - #5422 (CONTRIBUTING.md: fix broken triage link)
 - #5424 (Incorrect suspicious_op_assign_impl)
 - #5425 (Ehance opt_as_ref_deref lint.)

Failed merges:

 - #5411 (Downgrade implicit_hasher to pedantic)
 - #5428 (Move cognitive_complexity to nursery)

r? @ghost

changelog: rollup

Rollup of 11 pull requests

Successful merges:

 - #5406 (Fix update_lints)
 - #5409 (Downgrade let_unit_value to pedantic)
 - #5410 (Downgrade trivially_copy_pass_by_ref to pedantic)
 - #5412 (Downgrade inefficient_to_string to pedantic)
 - #5415 (Add new lint for `Result<T, E>.map_or(None, Some(T))`)
 - #5417 (Update doc links and mentioned names in docs)
 - #5419 (Downgrade unreadable_literal to pedantic)
 - #5420 (Downgrade new_ret_no_self to pedantic)
 - #5422 (CONTRIBUTING.md: fix broken triage link)
 - #5424 (Incorrect suspicious_op_assign_impl)
 - #5425 (Ehance opt_as_ref_deref lint.)

Failed merges:

 - #5345 (Add lint for float in array comparison)
 - #5411 (Downgrade implicit_hasher to pedantic)
 - #5428 (Move cognitive_complexity to nursery)

r? @ghost

changelog: rollup