Use apt to install packages on Debian and related distros

Apt is the modern recommended command to use when installing packages
these days on Debian and related distributions [1][2][3].  Convert the
Ubuntu GitLab CI jobs and update the README accordingly.

[1] What is the difference between apt and apt-get?
    https://askubuntu.com/questions/445384/what-is-the-difference-between-apt-and-apt-get
[2] Difference Between apt and apt-get Explained
    https://itsfoss.com/apt-vs-apt-get-difference/
[2] Apt and Apt-get - Which One to Use
    https://linoxide.com/apt-and-apt-get-which-one-to-use/

.gitlab-ci.yml

@@ -42,16 +42,14 @@ stages:
     - cat /proc/version
     - cat /etc/os-release
     - export DEBIAN_FRONTEND=noninteractive
-    - apt-get update
-    - apt-get install -y build-essential gnome-common yelp-tools
-                         libglib2.0-dev-bin uuid-dev libparted-dev
-                         libgtkmm-3.0-dev policykit-1
+    - apt update
+    - apt install -y build-essential gnome-common yelp-tools libglib2.0-dev-bin
+                     uuid-dev libparted-dev libgtkmm-3.0-dev policykit-1
     # Extra packages only needed during the test stage.
-    - apt-get install -y btrfs-progs e2fsprogs exfatprogs f2fs-tools dosfstools
-                         mtools hfsutils hfsprogs jfsutils util-linux
-                         cryptsetup-bin dmsetup lvm2 nilfs-tools ntfs-3g
-                         reiser4progs reiserfsprogs udftools xfsprogs xfsdump
-                         xvfb kmod
+    - apt install -y btrfs-progs e2fsprogs exfatprogs f2fs-tools dosfstools
+                     mtools hfsutils hfsprogs jfsutils util-linux cryptsetup-bin
+                     dmsetup lvm2 nilfs-tools ntfs-3g reiser4progs reiserfsprogs
+                     udftools xfsprogs xfsdump xvfb kmod
 
 .build_stage_template: &build_stage_definition
   stage: build

README

@@ -62,7 +62,7 @@ a. Pre-built Binary
 
       Debian or Ubuntu
       ----------------
-      sudo apt-get install gparted
+      sudo apt install gparted
 
       Fedora
       ------
@@ -104,11 +104,11 @@ b. Building from Source
    On Debian or Ubuntu, these dependencies may be obtained by running
    one of the following commands:
      Either;
-      sudo apt-get build-dep gparted
+      sudo apt build-dep gparted
      Or;
-      sudo apt-get install build-essential gnome-common yelp-tools \
-                           libglib2.0-dev uuid-dev libparted-dev \
-                           libgtkmm-3.0-dev
+      sudo apt install build-essential gnome-common yelp-tools \
+                       libglib2.0-dev uuid-dev libparted-dev \
+                       libgtkmm-3.0-dev
 
    On Fedora, these dependencies may be obtained by running the
    following command as root: