SIGSUM.md: update policy for v1.3.0

Updates #617

Author: FiloSottile

README.md

@@ -151,7 +151,7 @@ On Windows, Linux, macOS, and FreeBSD you can use the pre-built binaries.
 
 ```
 https://dl.filippo.io/age/latest?for=linux/amd64
-https://dl.filippo.io/age/v1.1.1?for=darwin/arm64
+https://dl.filippo.io/age/v1.3.0?for=darwin/arm64
 ...
 ```
 

SIGSUM.md

@@ -1,7 +1,7 @@
-If you download the pre-built binaries, you can check their
+If you download the pre-built binaries of version v1.2.0+, you can check their
 [Sigsum](https://www.sigsum.org) proofs, which are like signatures with extra
 transparency: you can cryptographically verify that every proof is logged in a
-public append-only log, so you can hold the age project accountable for every
+public append-only log, so the age project can be held accountable for every
 binary release we ever produced. This is similar to what the [Go Checksum
 Database](https://go.dev/blog/module-mirror-launch) provides.
 
@@ -10,20 +10,13 @@ cat << EOF > age-sigsum-key.pub
 ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIM1WpnEswJLPzvXJDiswowy48U+G+G1kmgwUE2eaRHZG
 ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIAz2WM5CyPLqiNjk7CLl4roDXwKhQ0QExXLebukZEZFS
 EOF
-cat << EOF > sigsum-trust-policy.txt
-log 154f49976b59ff09a123675f58cb3e346e0455753c3c3b15d465dcb4f6512b0b https://poc.sigsum.org/jellyfish
-witness poc.sigsum.org/nisse 1c25f8a44c635457e2e391d1efbca7d4c2951a0aef06225a881e46b98962ac6c
-witness rgdd.se/poc-witness  28c92a5a3a054d317c86fc2eeb6a7ab2054d6217100d0be67ded5b74323c5806
-group  demo-quorum-rule all poc.sigsum.org/nisse rgdd.se/poc-witness
-quorum demo-quorum-rule
-EOF
 
-curl -JLO "https://dl.filippo.io/age/v1.2.0?for=darwin/arm64"
-curl -JLO "https://dl.filippo.io/age/v1.2.0?for=darwin/arm64&proof"
+curl -JLO "https://dl.filippo.io/age/v1.3.0?for=darwin/arm64"
+curl -JLO "https://dl.filippo.io/age/v1.3.0?for=darwin/arm64&proof"
 
-go install sigsum.org/sigsum-go/cmd/sigsum-verify@v0.8.0
-sigsum-verify -k age-sigsum-key.pub -p sigsum-trust-policy.txt \
-    age-v1.2.0-darwin-arm64.tar.gz.proof < age-v1.2.0-darwin-arm64.tar.gz
+go install sigsum.org/sigsum-go/cmd/sigsum-verify@v0.13.1
+sigsum-verify -k age-sigsum-key.pub -P sigsum-generic-2025-1 \
+    age-v1.3.0-darwin-arm64.tar.gz.proof < age-v1.3.0-darwin-arm64.tar.gz
 ```
 
 You can learn more about what's happening above in the [Sigsum