[RISCV] Update Zfa extension version to 1.0 (#67964)

The Zfa specification was recently ratified
<https://wiki.riscv.org/display/HOME/Recently+Ratified+Extensions>. This
commit bumps the version to 1.0, but leaves it as an experimental
extension (to be done in a follow-on patch), so reviews can focus on
confirming there haven't been spec changes we have missed (which as
noted below, is more difficult than usual).

Because the development of the Zfa spec overlapped with the transition
of riscv-isa-manual from LaTeX to AsciiDoc, it's more difficult than
usual to confirm version changes. The linked PDF in RISCVUsage is for
some reason a 404. Key commit histories to review are:
* Changes to zfa.adoc on the main branch
<https://github.com/riscv/riscv-isa-manual/commits/main/src/zfa.adoc>
* Changes to zfa.tex on the now defunct latex branch
<https://github.com/riscv/riscv-isa-manual/commits/latex/src/zfa.tex>

From reviewing these, I believe there have been no changes to the spec
since version 0.1/0.2 (sadly the AsciiDoc and LaTeX versions of the spec
are inconsistent about version numbering).