Update AliasResolver to work with versions

What does this MR do and why?

Add alias_version to Import::SourceUserPlaceholderReference, so that when encountering old data, we can correctly map it to the new models/columns without any ambiguity due to changes in the schema.

MR acceptance checklist

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How to set up and validate locally

1. Try with the current version:

ref = Import::SourceUserPlaceholderReference.new(model: "Note", alias_version: 1, user_reference_column: "author_id")
ref.aliased_model # => Note
ref.aliased_user_reference_column # => "author_id"

2. Update ALIASES in lib/import/placeholder_references/alias_resolver.rb with a few different settings.

For example, apply this patch:

diff --git a/lib/import/placeholder_references/alias_resolver.rb b/lib/import/placeholder_references/alias_resolver.rb
index f8569735c215..7f46c54a34d0 100644
--- a/lib/import/placeholder_references/alias_resolver.rb
+++ b/lib/import/placeholder_references/alias_resolver.rb
@@ -13,6 +13,10 @@ module AliasResolver
           1 => {
             model: Note,
             columns: { "author_id" => "author_id" }
+          },
+          2 => {
+            model: MergeRequest,
+            columns: { "author_id" => "user_id" }
           }
         },
         "MergeRequest" => {

3. Run the commands again to see how it changes with a different version

ref = Import::SourceUserPlaceholderReference.new(model: "Note", alias_version: 2, user_reference_column: "author_id")
ref.aliased_model # => MergeRequest
ref.aliased_user_reference_column # => "user_id"

Related to #478501 (closed)