[FIX] web: repair some keynav in dropdowns

ENCOUNTERED ISSUE:
Unable to close a dropdown with ESC when the focus has been given
to a select element inside the dropdown.

TO REPRODUCE:
i.e. in the searchpanel, go to "FilterMenu > Add Custom Filter" and
open the field select element.
Press ESC once: the field select element closes just fine.
Press ESC a 2nd time: we expect the dropdown to close but nothing occur.

REASON EXPLANATION:
Since 84715436d our Dropdown component uses the 'dropdown-menu'
bootstrap class.
But under some conditions bootstrap stops the keydown events propagation
when the event.target matches the query selector '.dropdown-menu'.

IMPLEMENTED FIX:
This commit refines this selector directly inside the bootstrap library.
This is admittedly a bad practice but:
- this modification is trivial and sensible
- the modified selector is only used locally
  in the bootstrap's 'dropdown.js' file
- the added test should prevent the lost of
  this customization in the future

task-2764821

Part-of: odoo/odoo#84969

addons/web/static/lib/bootstrap/js/dropdown.js

@@ -111,7 +111,7 @@
   var Selector = {
     DATA_TOGGLE: '[data-toggle="dropdown"]',
     FORM_CHILD: '.dropdown form',
-    MENU: '.dropdown-menu',
+    MENU: '.dropdown-menu:not(.o-dropdown--menu)', // Odoo fix task-2764821
     NAVBAR_NAV: '.navbar-nav',
     VISIBLE_ITEMS: '.dropdown-menu .dropdown-item:not(.disabled):not(:disabled)'
   };

addons/web/static/tests/core/dropdown_tests.js

@@ -665,6 +665,37 @@ QUnit.module("Components", ({ beforeEach }) => {
         assert.verifySteps(["1", "2"], "items should have been selected in this order");
     });
 
+    QUnit.test("dropdowns keynav is not impacted by bootstrap", async (assert) => {
+        class Parent extends Component {}
+        Parent.components = { Dropdown };
+        Parent.template = xml`
+            <Dropdown startOpen="true">
+                <select><option>foo</option></select>
+            </Dropdown>
+        `;
+        env = await makeTestEnv();
+        await mount(Parent, target, { env });
+        assert.containsOnce(target, ".dropdown-menu", "menu is opened at start");
+        const menu = target.querySelector(".dropdown-menu");
+
+        // This class presence makes bootstrap ignore the below event
+        assert.hasClass(menu, "o-dropdown--menu");
+
+        const select = menu.querySelector("select");
+        const ev = new KeyboardEvent("keydown", {
+            bubbles: true,
+            // Define the ESC key with standard API (for hotkey_service)
+            key: "Escape",
+            code: "Escape",
+            // Define the ESC key with deprecated API (for bootstrap)
+            keyCode: 27,
+            which: 27,
+        });
+        select.dispatchEvent(ev);
+        await nextTick();
+        assert.containsNone(target, ".dropdown-menu", "menu is now closed");
+    });
+
     QUnit.test("props toggler='parent'", async (assert) => {
         class Parent extends Component {}
         Parent.template = xml`