Update rule selection to respect preview mode (#7195)

## Summary

<!-- What's the purpose of the change? What does it do, and why? -->

Extends work in #7046 (some relevant discussion there)

Changes:

- All nursery rules are now referred to as preview rules
- Documentation for the nursery is updated to describe preview
- Adds a "PREVIEW" selector for preview rules
- This is primarily to allow `--preview --ignore PREVIEW --extend-select
FOO001,BAR200`
- Using `--preview` enables preview rules that match selectors

Notable decisions:

- Preview rules are not selectable by their rule code without enabling
preview
- Retains the "NURSERY" selector for backwards compatibility
- Nursery rules are selectable by their rule code for backwards
compatiblity

Additional work:

- Selection of preview rules without the "--preview" flag should display
a warning
- Use of deprecated nursery selection behavior should display a warning
- Nursery selection should be removed after some time

## Test Plan

<!-- How was it tested? -->

Manual confirmation (i.e. we don't have an preview rules yet just
nursery rules so I added a preview rule for manual testing)

New unit tests

---------

Co-authored-by: Charlie Marsh <charlie.r.marsh@gmail.com>

crates/ruff_dev/src/generate_docs.rs

@@ -43,13 +43,10 @@ pub(crate) fn main(args: &Args) -> Result<()> {
                 output.push('\n');
             }
 
-            if rule.is_nursery() {
-                output.push_str(&format!(
-                    r#"This rule is part of the **nursery**, a collection of newer lints that are
-still under development. As such, it must be enabled by explicitly selecting
-{}."#,
-                    rule.noqa_code()
-                ));
+            if rule.is_preview() {
+                output.push_str(
+                    r#"This rule is in preview and is not stable. The `--preview` flag is required for use."#,
+                );
                 output.push('\n');
                 output.push('\n');
             }