chore: bump twmb/avro to v1.8.0

No code changes are required. rpk only uses Parse, SchemaCache.Parse,
Decode, Encode, DecodeJSON and EncodeJSON, none of which changed.

v1.8.0 changes how decimal logical types are encoded to JSON, which is
visible in `rpk topic consume` output:

    v1.7.2:  {"d":1094795.586}
    v1.8.0:  {"d":"AAAB"}

The new form is the encoding required by the Avro spec: a decimal is
bytes, and JSON-encoded bytes are a string of the raw byte values. Here
the unscaled value 1094795586 is 0x41414142, which is ASCII "AAAB".
fastavro emits byte-identical output; the old numeric form was a
divergence from the spec.