I can't verify this signature

You need the original MIME-encoded version of the mail to successfully verify manually i.e.

to-verify-txt
8X---------
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: quoted-printable

Hello!

We are pleased to announce the availability of Libgcrypt version 1.11.0.
[...]

=2D-=20
The pioneers of a warless world are the youth that
refuse military service. - A. Einstein
8X---------

(Tested with the version on gmane, which is available via NNTP.)

The signature on the tarball verifies as documented:
ametzler@argenau:/tmp$ gpg --verify --keyring /tmp/signature_key.asc libgcrypt-1.11.0.tar.bz2.sig libgcrypt-1.11.0.tar.bz2
gpg: Signature made Mi 19 Jun 2024 11:31:18 CEST
gpg: using EDDSA key 6DAA6E64A76D2840571B4902528897B826403ADA
gpg: Good signature from "Werner Koch (dist signing 2020)" [unknown]
gpg: WARNING: This key is not certified with a trusted signature!
gpg: There is no indication that the signature belongs to the owner.
Primary key fingerprint: 6DAA 6E64 A76D 2840 571B 4902 5288 97B8 2640 3ADA