tests: Use PGP keys without SHA-1

[ppisar]

Tests failed on RHEL 10 where SHA-1 is disabled in a DEFAULT crypto
policy and where librepo is configured to use rpm-sequoia which
respects the crypto policy (in contrast to gpgme):

    1: ======================================================================
    1: FAIL: test_rawkey2infos (tests.test_crypto.CryptoTest.test_rawkey2infos)
    1: ----------------------------------------------------------------------
    1: Traceback (most recent call last):
    1:   File "/home/test/rhel/dnf/dnf-4.20.0/tests/test_crypto.py", line 75, in test_rawkey2infos
    1:     self.assertEqual(info.userid, 'Dandy Fied <[email protected]>')
    1: AssertionError: '' != 'Dandy Fied <[email protected]>'
    1: + Dandy Fied <[email protected]>

The root cause was that tests/keys/key.pub used the SHA-1 digest algorithm.

This patch replaces that key with a 4096-bit RSA key signed using
SHA-384 digest algorithm.

Resolves: https://issues.redhat.com/browse/RHEL-50218

[ppisar]

For you information, Fedora is not affected because it still allows SHA-1 for signing packages.