Migrate to rsa 0.6 and chacha20poly1305 0.10

[str4d]

This brings us onto Rust Crypto crates with MSRV 1.56 or lower.

Closes #304.

[codecov]

Codecov Report

Merging #303 (885b90f) into main (a86fb04) will decrease coverage by 0.11%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #303      +/-   ##
==========================================
- Coverage   37.74%   37.63%   -0.12%     
==========================================
  Files          34       34              
  Lines        2999     3000       +1     
==========================================
- Hits         1132     1129       -3     
- Misses       1867     1871       +4     

Impacted Files	Coverage Δ
age-core/src/primitives.rs	92.85% <ø> (ø)
age/src/primitives/stream.rs	61.60% <ø> (-0.79%)	⬇️
age/src/ssh/identity.rs	51.88% <ø> (ø)
age/src/ssh/recipient.rs	67.85% <ø> (ø)
age/src/primitives.rs	50.00% <0.00%> (-8.34%)	⬇️
age/src/identity.rs	65.62% <0.00%> (-1.57%)	⬇️
age/src/primitives/armor.rs	41.47% <0.00%> (-0.20%)	⬇️
age/src/format.rs	50.54% <0.00%> (ø)
age-core/src/format.rs	61.94% <0.00%> (+0.74%)	⬆️
... and 1 more

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