16c157d qt, refactor: Use better QMenu::addAction overloaded function (Hennadii Stepanov)
7931175 qt: Do not assign Alt+<KEY> shortcuts to context menu actions (Hennadii Stepanov)
963e120 qt: Drop menu separator that separates nothing (Hennadii Stepanov)
1398a65 qt, refactor: Make AddressBookPage::deleteAction a local variable (Hennadii Stepanov)

Pull request description:

  This PR:
  1. removes useless `Alt` + `<KEY>` shortcuts from context menu items
  2. replaces 3 lines of code with the only call of [`QMenu::addAction`](https://doc.qt.io/qt-5/qmenu.html#addAction-5) for each context menu item (it became possible since bitcoin/bitcoin#21286 was merged)
  3. makes other minor cleanups

  No behavior change.

ACKs for top commit:
  kristapsk:
    ACK 16c157d
  promag:
    Code review ACK 16c157d. Nice code cleanup that takes advantage of more recent Qt API.
  jarolrod:
    ACK 16c157d

Tree-SHA512: e5555fe957058cc67b351aaf9f09fe3635edb2d07a2223d3093913a25607ae538f0a2fde84c0b0cd43e7475b248949548eb4a5d4b21d8f7391fa2fa8541c04ff

…connections

cdbc2bd qt: Use template function qOverload in signal-slot connections (Hennadii Stepanov)

Pull request description:

  A nice template function [`qOverload`](https://doc.qt.io/qt-5/qtglobal.html#qOverload) is available for us now (bitcoin/bitcoin#20413, bitcoin/bitcoin#21286).

  Its usage makes code much more readable.

  This PR does not change behavior.

ACKs for top commit:
  Talkless:
    utACK cdbc2bd.
  promag:
    Code review ACK cdbc2bd.

Tree-SHA512: 72002aa646b1a79bab62d498825b3f245dc7ebdc189280f8bd3b4076e1bb50be8802c02bc872ff6f70c1ea81faec66d3bec36471119dd98c9e70d87b990396ae

956f732 build: Bump minimum Qt version to 5.11.3 (Hennadii Stepanov)
e22d10b ci: Switch from bionic to buster (Hennadii Stepanov)

Pull request description:

  The current minimum Qt version is 5.9.5 which has been set in bitcoin/bitcoin#21286.

  Distro support:
  - centos 7 -- unsupported since bitcoin/bitcoin#23511
  - centos 8 -- [5.15.2](http://mirror.centos.org/centos/8/AppStream/x86_64/os/Packages/qt5-qtbase-5.15.2-3.el8.x86_64.rpm)
  - buster -- [5.11.3](https://packages.debian.org/buster/libqt5core5a)
  - bullseye  -- [5.15.2](https://packages.debian.org/bullseye/libqt5core5a)
  - _bionic_ -- [5.9.5](https://packages.ubuntu.com/bionic/libqt5core5a)
  - focal -- [5.12.8](https://packages.ubuntu.com/focal/libqt5core5a)

  As another Ubuntu LTS is coming soon, it seems unreasonable to stick to Qt 5.9 which support [ended](https://www.qt.io/blog/2017/06/07/renewed-qt-support-services) on 2020-05-31. Anyway, it's still possible to build Bitcoin Core GUI with depends on bionic system.

  Bumping the minimum Qt version allows to make code safer and more reliable, e.g.:
  - functor-parameter overload of [`QMetaObject::invokeMethod`](https://doc.qt.io/qt-5/qmetaobject.html#invokeMethod-4)
  - fixed https://bugreports.qt.io/browse/QTBUG-10907

  An example of the patch using the functor-overload of `QMetaObject::invokeMethod`:
  ```diff
  --- a/src/qt/walletmodel.cpp
  +++ b/src/qt/walletmodel.cpp
  @@ -349,7 +349,7 @@ bool WalletModel::changePassphrase(const SecureString &oldPass, const SecureStri
   static void NotifyUnload(WalletModel* walletModel)
   {
       qDebug() << "NotifyUnload";
  -    bool invoked = QMetaObject::invokeMethod(walletModel, "unload");
  +    bool invoked = QMetaObject::invokeMethod(walletModel, &WalletModel::unload);
       assert(invoked);
   }

  ```
  It uses the same new syntax as signal-slot connection with compile-time check. Also see bitcoin/bitcoin#16348.

  This PR is intended to be merged early [after](bitcoin/bitcoin#22969) branching `23.x` off.

ACKs for top commit:
  MarcoFalke:
    cr ACK 956f732
  fanquake:
    ACK 956f732

Tree-SHA512: 3d652bcdcd990ce785ad412ed70234d4f27743895e535a53ed44b35d4afc3052e066c4c84f417e30bc53d0a3dd9ebed62444c57b7c765cb1e9aa687fbf866877