virtual/mta: track upstream version instead of a frozen r902

virtual/mta: track upstream version instead of a frozen r902

r1/r902 were pinned artificially high to always outrank the main
tree's virtual/mta, but that means any real upstream bump would stay
permanently invisible. Replaced with mta-1-r2, matching the main
tree's current version exactly plus our mail-mta/smailq[mta] addition.

Version-matching alone doesn't actually make this override win here:
with --getbinpkg on, an identically-versioned binhost binary for
::gentoo outranks building this ebuild regardless of repo priority.
Needs virtual/mta::gentoo in package.mask to take effect; documented
in the ebuild since it's not otherwise discoverable.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>