[improve][build] Upgrade lightproto gradle plugin to 0.8.0

[merlimat]

Motivation

LightProto v0.8.0 is a systematic performance pass over the generated serialization/deserialization code, benchmarked directly on Pulsar's protocol messages (vs 0.7.3):

Benchmark (ops/µs)	v0.7.3	v0.8.0	Δ
MessageMetadata deserialize	14.3	23.6	+66%
BaseCommand deserialize	33.2	46.7	+41%
BaseCommand serialize	19.5	43.3	+122%
MessageMetadata serialize	12.0	14.5	+21%

Main changes in the generated code:

• O(1) clear() with presence-guarded getters (also fixes stale-child reachability after clearX() on a message field, and copyFrom() now follows protobuf merge semantics for implicit-presence fields)
• Serialization composes into a plain byte[] instead of per-field ByteBuf writes; writeTo(CompositeByteBuf) now works
• Presence-bit-driven traversal for union-like messages (e.g. BaseCommand with ~50 optional sub-commands)
• Removal of sun.misc.Unsafe from the write hot loop and the ASCII scan (JDK 24+ taxes each call, and it is on the removal track)

Modifications

• Bump lightproto from 0.7.3 to 0.8.0 in gradle/libs.versions.toml; all modules applying the plugin pick it up via the version catalog.