man: systemd-stub places PCR public key in file 'tpm2-pcr-public-key.pem', stick...

man: systemd-stub places PCR public key in file 'tpm2-pcr-public-key.pem', stick to that name across the board

systemd-stub provides the signing key for TPM2 signed PCR policies in a
file tpm2-pcr-public-key.pem to userspace. Hence, to clarify that this
is the same key as used when signing via "systemd-measure", let's rename
it in the docs like that.

Also rename the private key to tpm2-pcr-private-key.pem, to keep the
symmetry.

With this we should universally stick to this nomenclature:

1. tpm2-pcr-public-key.pem   ← public part of signing key
2. tpm2-pcr-private-key.pem  ← private part of signing key
3. tpm2-pcr-signature.json   ← signature file made with key pair

Inspired by: #34069