Perform a check of required packages befor updating pihole

[yubiuser]

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• I have made only one major change in my proposed changes.
• I have commented my proposed changes within the code.
• I have tested my proposed changes, and have included unit tests where possible.
• I am willing to help maintain this change if there are issues with it later.
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• It is compatible with the EUPL 1.2 license
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Follow up of #4274

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For discussion.

What does this PR aim to accomplish?:
Fixes rare edge cases like #2866

If users have removed packages that are part of INSTALLER_DEPS (e.g. git), pihole -up will fail.
This PR re-uses the install_dependent_packages "${INSTALLER_DEPS[@]}" from the installer script to check, and if necessary, install the missing packages.

rockpi@rockpi-4b:~$ sudo apt remove git
[sudo] password for rockpi: 
Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following packages were automatically installed and are no longer required:
  git-man liberror-perl
Use 'sudo apt autoremove' to remove them.
The following packages will be REMOVED:
  git
0 upgraded, 0 newly installed, 1 to remove and 5 not upgraded.
After this operation, 36.1 MB disk space will be freed.
Do you want to continue? [Y/n] y
(Reading database ... 41003 files and directories currently installed.)
Removing git (1:2.20.1-2+deb10u3) ...
rockpi@rockpi-4b:~$ pihole -up

  Error: Core Pi-hole repo is missing from system!
  Please re-run install script from https://pi-hole.net

rockpi@rockpi-4b:~$ sudo ./update.sh 
  [✓] Update local cache of available packages
  [i] Existing PHP installation detected : PHP version 7.3.29-1~deb10u1
  [i] Checking for git (will be installed)
  [✓] Checking for iproute2
  [✓] Checking for whiptail
  [i] Processing apt-get install(s) for: git, please wait...
---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Selecting previously unselected package git.
(Reading database ... 40250 files and directories currently installed.)
Preparing to unpack .../git_1%3a2.20.1-2+deb10u3_arm64.deb ...
Unpacking git (1:2.20.1-2+deb10u3) ...
Setting up git (1:2.20.1-2+deb10u3) ...
---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
  [i] Checking for updates...
  [i] Pi-hole Core:	update available
  [i] Web Interface:	up to date
  [i] FTL:		update available
  [i] Warning: You are using FTL from a custom branch (release/v5.9) and might be missing future releases.