message: doc: Update the used Qt version
  A bitcoin/bitcoin#23489 follow up.
diff: @@ -20,7 +20,7 @@ These are the dependencies currently used by Peercoin. You can find instructions
  | PCRE | | | | | [Yes](https://github.com/bitcoin/bitcoin/blob/master/depends/packages/qt.mk) |
  | Python (tests) | | [3.6](https://www.python.org/downloads) | | | |
  | qrencode | [3.4.4](https://fukuchi.org/works/qrencode) | | No | | |
  -| Qt | [5.12.11](https://download.qt.io/official_releases/qt/) | [5.9.5](https://github.com/bitcoin/bitcoin/issues/20104) | No | | |
  +| Qt | [5.15.2](https://download.qt.io/official_releases/qt/) | [5.9.5](https://github.com/bitcoin/bitcoin/issues/20104) | No | | |
  | SQLite | [3.32.1](https://sqlite.org/download.html) | [3.7.17](https://github.com/bitcoin/bitcoin/pull/19077) | | | |
  | XCB | | | | | [Yes](https://github.com/bitcoin/bitcoin/blob/master/depends/packages/qt.mk) (Linux only) |
  | systemtap ([tracing](tracing.md))| [4.5](https://sourceware.org/systemtap/ftp/releases/) | | | | |
  