style(delta): align @impl with ash_postgres.migrate.ex convention

Review pass: codebase precedent for the `run/1` annotation on a
Mix.Task with logic (not a pure delegation shim) is `@impl true`
in ash_postgres.migrate.ex. Match that in migrate_snapshots and
squash_snapshots, replacing `@impl Mix.Task` which I'd reached for
on autopilot.

Remaining convention divergence — shim-style tasks like
ash_postgres.create.ex use `@doc false` instead of `@impl`, which
is its own established pattern for a different class of task.
Neither task here fits that class.

Author: barnabasJ

lib/mix/tasks/ash_postgres.migrate_snapshots.ex

@@ -46,7 +46,7 @@ defmodule Mix.Tasks.AshPostgres.MigrateSnapshots do
     quiet: :boolean
   ]
 
-  @impl Mix.Task
+  @impl true
   def run(args) do
     {opts, _} = OptionParser.parse!(args, strict: @switches)
 

lib/mix/tasks/ash_postgres.squash_snapshots.ex

@@ -55,7 +55,7 @@ defmodule Mix.Tasks.AshPostgres.SquashSnapshots do
   * `--include-dev` - include `*_dev.json` files in the squash (default: skip them). Delta-mode squash aborts if dev files are present and this flag is not set.
   """
 
-  @impl Mix.Task
+  @impl true
   def run(args) do
     {opts, []} = OptionParser.parse!(args, strict: @switches)