BIP125: update status to Final

Discussing #1692 with @luke-jr, realized that we may not have followed BIP2 process when updating BIP125 from Proposed to Obsolete.

Per BIP2:

"A Proposed BIP may progress to Final only when specific criteria reflecting real-world adoption has occurred."

"When a Final BIP is no longer relevant, its status may be changed to Replaced or Obsolete (which is equivalent to Replaced). This change must also be objectively verifiable and/or discussed."

BIP125 saw real-world adoption, so it would make sense to update it from Proposed to Final. But arguably not from Proposed to Replaced/Obsolete as done in #1692, as it was not final, not replaced by another BIP, and still relevant (also supported in at least one bitcoin implementation).