Auto merge of #8260 - taiki-e:mutex_atomic, r=llogiq

Downgrade mutex_atomic to nursery

See #1516 and #4295.

There are suggestions about removing this lint from the default warned lints in both issues.
Also, [`mutex_integer`](https://rust-lang.github.io/rust-clippy/master/index.html#mutex_integer) lint that has the same problems as this lint is in `nursery` group.

changelog: Moved [`mutex_atomic`] to `nursery`

clippy_lints/src/lib.register_all.rs

@@ -206,7 +206,6 @@ store.register_group(true, "clippy::all", Some("clippy_all"), vec![
     LintId::of(mut_key::MUTABLE_KEY_TYPE),
     LintId::of(mut_mutex_lock::MUT_MUTEX_LOCK),
     LintId::of(mut_reference::UNNECESSARY_MUT_PASSED),
-    LintId::of(mutex_atomic::MUTEX_ATOMIC),
     LintId::of(needless_arbitrary_self_type::NEEDLESS_ARBITRARY_SELF_TYPE),
     LintId::of(needless_bool::BOOL_COMPARISON),
     LintId::of(needless_bool::NEEDLESS_BOOL),

clippy_lints/src/lib.register_nursery.rs

@@ -15,6 +15,7 @@ store.register_group(true, "clippy::nursery", Some("clippy_nursery"), vec![
     LintId::of(let_if_seq::USELESS_LET_IF_SEQ),
     LintId::of(missing_const_for_fn::MISSING_CONST_FOR_FN),
     LintId::of(mutable_debug_assertion::DEBUG_ASSERT_WITH_MUT_CALL),
+    LintId::of(mutex_atomic::MUTEX_ATOMIC),
     LintId::of(mutex_atomic::MUTEX_INTEGER),
     LintId::of(non_send_fields_in_send_ty::NON_SEND_FIELDS_IN_SEND_TY),
     LintId::of(nonstandard_macro_braces::NONSTANDARD_MACRO_BRACES),

clippy_lints/src/lib.register_perf.rs

@@ -19,7 +19,6 @@ store.register_group(true, "clippy::perf", Some("clippy_perf"), vec![
     LintId::of(methods::SINGLE_CHAR_PATTERN),
     LintId::of(methods::UNNECESSARY_TO_OWNED),
     LintId::of(misc::CMP_OWNED),
-    LintId::of(mutex_atomic::MUTEX_ATOMIC),
     LintId::of(redundant_clone::REDUNDANT_CLONE),
     LintId::of(slow_vector_initialization::SLOW_VECTOR_INITIALIZATION),
     LintId::of(stable_sort_primitive::STABLE_SORT_PRIMITIVE),

clippy_lints/src/mutex_atomic.rs

@@ -38,7 +38,7 @@ declare_clippy_lint! {
     /// ```
     #[clippy::version = "pre 1.29.0"]
     pub MUTEX_ATOMIC,
-    perf,
+    nursery,
     "using a mutex where an atomic value could be used instead"
 }