Lower the default allowed RSA key size to ensure that (most) keys generated in
v4 without WebCrypto are supported (see #1336).

…ed algorithms (#1395)

Breaking changes:
- throw error on key generation if the requested public key algorithm is
included in `config.rejectPublicKeyAlgorithms`;
- add `config.rejectCurves` to blacklist a set of ECC curves, to prevent keys
using those curves from being generated, or being used to
encrypt/decrypt/sign/verify messages.
By default, `config.rejectCurves` includes the brainpool curves
(`brainpoolP256r1`, `brainpoolP384r1`, `brainpoolP512r1`) and the Bitcoin curve
(`secp256k1`). This is because it's unclear whether these curves will be
standardised[1], and we prefer to blacklist them already, rather than introduce
a breaking change after release.

[1] https://gitlab.com/openpgp-wg/rfc4880bis/-/merge_requests/47#note_634199141

…fy` and `decrypt`