[WEBSITE] Use version number in the release page links

Summary:
Use version number instead of the latest symlink. This nullifies the
risk of being out of sync, and better match the release notes.

Test Plan:

make serve

Reviewers: #bitcoin_abc, PiRK, jasonbcox

Reviewed By: #bitcoin_abc, PiRK, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D8263