Add php version requirements file for future-proofing upgrades

[jerm]

There is a race condition in the upgrade.php file where it can't
currently know the hard requirements for the version it's upgrading to
until it does a git pull. By that time, it will have pulled new source
code that possibly relies on an incompatible version of php, leaving you
with a broken installation.

Example: You're running v6.2.x on PHP 7.4, which is fine. v7 requires
PHP 8.1 or 8.2, but we couldn't know that when we released v6. or v5 for
that matter. We could change the requirements in the most-recent v6
version of upgrade.php, but that doesn't help anyone who doesn't upgrade
first to the most recent v6.

With this change we'll keep a requirements file,
.upgrade_requirements.json, up to date in the root of the default
branch as the requirements to upgrade to HEAD change.

The upgrade.php file will query this file from Github at runtime, to
see if it's safe to pull down the rest of the repo, and warn you if not.

tl;dr, instead of hard-coding the php version requirements into
upgrade.php, we'll fetch the current ones from the live repo at runtime

This file is being added in a separate PR from the upgrade.php changes
so that testing of the script can be done against the live repo. Cart
and horse, and all that.

[what-the-diff]

PR Summary

• Added '.upgrade_requirements.json' File
  The developer team has included a new special file known as '.upgrade_requirements.json'. This file, similar to a checklist, keeps track of what is required from the server where our product runs.

• Insertion of Documentation Lines
  The team has added numerous documentation lines (DOC1, DOC2, DOC3, DOC4). This ensures that our programmers understand the code's inner workings and enhances productivity when future updates or changes are introduced.

• Upgraded PHP Minimum Version Requirement
  To make our product more robust and to utilize the latest language capabilities, the team upgraded the minimum supported PHP version to "8.1.0". It's like ensuring our product runs on the latest engine for the best performance.

• Update of PHP Maximum Version Restriction
  The developers have set a maximum boundary for PHP version (8.2), ensuring that the product works best within this range. It provides stability by preventing unintended consequences of using untested or unsupported language versions.

• PHP Non-support Version Determined
  The product will not work with PHP version "8.3.0". This is like specifying what fuel our product won't run on. This information is important to avoid potential setup issues.

• Updated Current Snipe-IT Version
  The current version of Snipe-IT, our core product, has been updated to version "6.3". This could include various improvements, feature additions, or bug fixes enacted since the previous release.

[snipe]

Do we also maybe want this on master?