[libc++][ranges][NFC] Mark the completed Ranges papers and issues as done.

The newly-completed papers:
- P0896R4 ("The One Ranges Proposal");
- P1243R4 ("Rangify New Algorithms");
- P1252R2 ("Ranges Design Cleanup");
- P1716R3 ("Range Comparison Algorithms Are Over-Constrained");
- P1871R1 ("Concept traits should be named after concepts");
- P2106R0 ("Alternative wording for GB315 and GB316").

Differential Revision: https://reviews.llvm.org/D131234