Rename applicability levels to Safe, Unsafe, and Display (#7843)

After working with the previous change in
https://github.com/astral-sh/ruff/pull/7821 I found the names a bit
unclear and their relationship with the user-facing API muddied. Since
the applicability is exposed to the user directly in our JSON output, I
think it's important that these names align with our configuration
options. I've replaced `Manual` or `Never` with `Display` which captures
our intent for these fixes (only for display). Here, we create room for
future levels, such as `HasPlaceholders`, which wouldn't fit into the
`Always`/`Sometimes`/`Never` levels.

Unlike https://github.com/astral-sh/ruff/pull/7819, this retains the
flat enum structure which is easier to work with.

crates/ruff_linter/src/message/diff.rs

@@ -53,9 +53,9 @@ impl Display for Diff<'_> {
 
         let message = match self.fix.applicability() {
             // TODO(zanieb): Adjust this messaging once it's user-facing
-            Applicability::Always => "Fix",
-            Applicability::Sometimes => "Suggested fix",
-            Applicability::Never => "Possible fix",
+            Applicability::Safe => "Fix",
+            Applicability::Unsafe => "Suggested fix",
+            Applicability::Display => "Possible fix",
         };
         writeln!(f, "ℹ {}", message.blue())?;