Raise minimum versions of browsers

For the following reasons:

- Support for CSS `gap`, so as to remove the need to
  fall back onto classic popup panel, and thus making
  it possible to fully remove long-ago deprecated
  classic panel (announced in 1.27.0)[1]:
  https://developer.mozilla.org/en-US/docs/Web/CSS/gap

- Support for dynamic import:
  https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/import

Firefox's minimum version is raised to 68 because it's
the oldest ESR version supporting the features above.
From now on, minimum version of Firefox will always be
an ESR one.

---

[1] https://github.com/gorhill/uBlock/releases/tag/1.27.0

platform/chromium/manifest.json

@@ -70,7 +70,7 @@
   },
   "incognito": "split",
   "manifest_version": 2,
-  "minimum_chrome_version": "61.0",
+  "minimum_chrome_version": "66.0",
   "name": "uBlock Origin",
   "options_ui": {
     "page": "dashboard.html",

platform/firefox/manifest.json

@@ -15,7 +15,7 @@
   "browser_specific_settings": {
     "gecko": {
       "id": "uBlock0@raymondhill.net",
-      "strict_min_version": "60.0"
+      "strict_min_version": "68.0"
     }
   },
   "commands": {

platform/opera/manifest.json

@@ -69,7 +69,7 @@
   },
   "incognito": "split",
   "manifest_version": 2,
-  "minimum_opera_version": "48.0",
+  "minimum_opera_version": "53.0",
   "name": "uBlock Origin",
   "options_page": "dashboard.html",
   "permissions": [

src/js/start.js

@@ -487,20 +487,11 @@ contextMenu.update();
 // default UI according to platform.
 if (
     browser.browserAction instanceof Object &&
-    browser.browserAction.setPopup instanceof Function
-) {
-    const env = vAPI.webextFlavor;
-    if (
-        µb.hiddenSettings.uiFlavor === 'classic' || (
-            µb.hiddenSettings.uiFlavor === 'unset' && (
-                env.soup.has('chromium') && env.major < 66 ||
-                env.soup.has('firefox') && env.major < 68
-            )
-        )
+    browser.browserAction.setPopup instanceof Function &&
+    µb.hiddenSettings.uiFlavor === 'classic'
 ) {
     browser.browserAction.setPopup({ popup: 'popup.html' });
 }
-}
 
 // https://github.com/uBlockOrigin/uBlock-issues/issues/717
 //   Prevent the extension from being restarted mid-session.