DEV: Change slugs/generate endpoint from GET to POST (#19984)

Followup on feedback on PR #19928
https://github.com/discourse/discourse/pull/19928#discussion_r1083687839,
it makes more sense to have this endpoint as a POST rather than
a GET.

spec/requests/slugs_controller_spec.rb

@@ -8,7 +8,7 @@ RSpec.describe SlugsController do
 
     context "when user not logged in" do
       it "returns a 403 error" do
-        get "/slugs/generate.json?name=#{name}"
+        post "/slugs.json", params: { name: name }
         expect(response.status).to eq(403)
       end
     end
@@ -17,7 +17,7 @@ RSpec.describe SlugsController do
       before { sign_in(current_user) }
 
       it "generates a slug from the name" do
-        get "/slugs/generate.json", params: { name: name }
+        post "/slugs.json", params: { name: name }
         expect(response.status).to eq(200)
         expect(response.parsed_body["slug"]).to eq(Slug.for(name, ""))
       end
@@ -26,15 +26,15 @@ RSpec.describe SlugsController do
         RateLimiter.enable
 
         stub_const(SlugsController, "MAX_SLUG_GENERATIONS_PER_MINUTE", 1) do
-          get "/slugs/generate.json?name=#{name}"
-          get "/slugs/generate.json?name=#{name}"
+          post "/slugs.json?name=#{name}"
+          post "/slugs.json?name=#{name}"
         end
 
         expect(response.status).to eq(429)
       end
 
       it "requires name" do
-        get "/slugs/generate.json"
+        post "/slugs.json"
         expect(response.status).to eq(400)
       end
 
@@ -42,7 +42,7 @@ RSpec.describe SlugsController do
         before { current_user.change_trust_level!(1) }
 
         it "returns a 403 error" do
-          get "/slugs/generate.json?name=#{name}"
+          post "/slugs.json?name=#{name}"
           expect(response.status).to eq(403)
         end
       end
@@ -51,7 +51,7 @@ RSpec.describe SlugsController do
         fab!(:current_user) { Fabricate(:admin) }
 
         it "generates a slug from the name" do
-          get "/slugs/generate.json", params: { name: name }
+          post "/slugs.json", params: { name: name }
           expect(response.status).to eq(200)
           expect(response.parsed_body["slug"]).to eq(Slug.for(name, ""))
         end