docs: Update Tendermint/Rust versions in READMEs

[thanethomson]

For the next v0.17.0-related release, we should probably bump these version numbers.

• Referenced an issue explaining the need for the change
• Updated all relevant documentation in docs
• Updated all code comments where relevant
• Wrote tests
• Updated CHANGELOG.md

[greg-szabo]

I'm ok with this but is there any reason why Rust 1.47 was specifically chosen? (I guess we only support the latest stable normally, is that it?)

[thanethomson]

Yes, I was wondering if we should rather just label it as stable instead of specifying the exact version. I think I'll change it to stable.

[codecov-io]

Codecov Report

Merging #642 into master will decrease coverage by 0.0%.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           master    #642     +/-   ##
========================================
- Coverage    42.7%   42.6%   -0.1%     
========================================
  Files         167     167             
  Lines       11613   11614      +1     
  Branches     2599    2602      +3     
========================================
  Hits         4959    4959             
- Misses       6362    6363      +1     
  Partials      292     292             

Impacted Files	Coverage Δ
light-client/src/evidence.rs	0.0% <0.0%> (ø)
light-client/src/components/io.rs	0.0% <0.0%> (ø)
light-client/src/builder/supervisor.rs	0.0% <0.0%> (ø)

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