Use `git filter-repo`

[hectorsector]

Describe the bug
The course currently uses git filter-branch which results in the following message from git:

❯ git filter-branch --index-filter "git rm -rf --cached --ignore-unmatch .env" HEAD
WARNING: git-filter-branch has a glut of gotchas generating mangled history
         rewrites.  Hit Ctrl-C before proceeding to abort, then use an
         alternative filtering tool such as 'git filter-repo'
         (https://github.com/newren/git-filter-repo/) instead.  See the
         filter-branch manual page for more details; to squelch this warning,
         set FILTER_BRANCH_SQUELCH_WARNING=1.
Proceeding with filter-branch...

To Reproduce
This is in Step 13: Remove historical reference to a previous .env file

Expected behavior
Git shouldn't complain if we're using the proper command.