Undocumented golang and git version requirements

**Describe the bug**
Git and golang have minimum version requirements to build gitleaks

**To Reproduce**
On a RHEL 7 server, git clone this repo and run "make build"

**Expected behavior**
I expect a successful build. Instead, the golang download fails on the standard git 1.8.3.
If an updated git is installed, it fails on golang 1.16 being too old, and requiring golang-1.17.

**Screenshots**
If applicable, add screenshots to help explain your problem.

**Basic Info (please complete the following information):**
- OS: RHEL 7
- Gitleaks Version: 8.6.0

**Additional context**
I was trying to test this on RHEL 7, which will definitely not work without component updates. Any external github repos from which content is yanked down by the build process will never be usable for "mock" or "koji" based builds for RHEL or Fedora deployment.

cc @zricethezav

Start by reviewing the build process invoked by `make build` and the project's existing documentation. Document the minimum Git and Go versions needed to build Gitleaks, and clarify the external download constraints described for RHEL 7. Done means users can find these requirements before attempting a build.

Written by the indexing model from the issue text.