Pin MSRV and the lower bound of some dependencies

[fjarri]

• pin MSRV to 1.65 in Cargo.toml
• add lower bound openssl=0.10.39 (compilation fails before that)
• add lower bound rand_core=0.6.4 (CryptoRngCore introduced; the previous 0.6 worked before because crypto-bigint had 0.6.4 in the dependencies, but since we're using that trait too, we should also bound it)

[codecov[bot]]

Codecov Report

Patch coverage has no change and project coverage change: -0.01% :warning:

Comparison is base (7d95cf6) 98.65% compared to head (d2500b6) 98.65%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #31 +/- ## ========================================== - Coverage 98.65% 98.65% -0.01% ========================================== Files 10 9 -1 Lines 1339 1338 -1 ========================================== - Hits 1321 1320 -1 Misses 18 18 ``` [see 1 file with indirect coverage changes](https://app.codecov.io/gh/entropyxyz/crypto-primes/pull/31/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=entropyxyz)

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