porters-handbook/testing: Pet vale

• Remove repocopy/repocopied as this concept no longer exists after git migration
• Fix semantic line breaks
• Use the term poudriere as oposed to Poudriere recommended by the authors of poudriere
• Remove optional configuration snippets like ZROOTFS/FREEBSD_HOST/SVN_HOST from poudriere.conf config file
• Remove references to SVN/subversion and replace with relevant git terms
• Remove references to SVN/subversion CAUTION/NOTE/TIP
• Remove pronouns like you/they
• Replace references to EOL 12.3-STABLE with 12.4-STABLE
• Remove references to older poudriere version 3.1.20

Approved by: portmgr bcr(mentor)
Differential Revision: https://reviews.freebsd.org/D40134