Merge bitcoin-core/gui#40: Clarify block height label

b6dcc6d741 gui: Clarify block height label (Hennadii Stepanov)

Pull request description:

  Prefer "block height" instead of "number of blocks".

  This was done while testing https://github.com/bitcoin/bitcoin/pull/16981.

ACKs for top commit:
  michaelfolkson:
    ACK b6dcc6d741. I don't think there are any other obvious examples in the GUI where "block height" should replace "number of blocks" except for translations.
  MarcoFalke:
    cr ACK b6dcc6d741

Tree-SHA512: ec3b48c1af5d613ed657ad51f2caddea774376736ecc02343d54518986e35ec37f1745b059814b5be92b5e5c2bb2970d17159b24c6e88b9316803d4de5327c31

src/qt/forms/debugwindow.ui

@@ -290,7 +290,7 @@
        <item row="11" column="0">
         <widget class="QLabel" name="label_3">
          <property name="text">
-          <string>Current number of blocks</string>
+          <string>Current block height</string>
          </property>
         </widget>
        </item>