Merge GOSSIP_MAX_SIZE and MAX_CHUNK_SIZE into MAX_PAYLOAD_SIZE

[etan-status]

In upstream specs the preset values for indicating max payload sizes have been merged into a single MAX_PAYLOAD_SIZE constant.

[tersec]

[2025-02-24T19:41:56.992Z]        > /build/source/beacon_chain/rpc/rest_config_api.nim(219, 29) Error: undeclared identifier: 'GOSSIP_MAX_SIZE'
[2025-02-24T19:41:56.992Z]        > candidates (edit distance, scope distance); see '--spellSuggest':
[2025-02-24T19:41:56.992Z]        >  (6, 6): 'GossipState'
[2025-02-24T19:41:56.992Z]        >  (6, 6): 'formatSize'
[2025-02-24T19:41:56.992Z]        >  (6, 6): 'maxSize'
[2025-02-24T19:41:56.992Z]        > make: *** [Makefile:441: nimbus_beacon_node] Error 1
[2025-02-24T19:41:56.992Z]        For full logs, run 'nix log /nix/store/yd138ibm9m5hh8a8mfr5bz71ds0zw14k-nimbus-eth2-25.2.0-991d0993.drv'.

[etan-status]

Fixed that one, have forgotten to commit one of the changes :)

[github-actions]

Unit Test Results

       15 files  ±0    2 614 suites  ±0   1h 13m 57s ⏱️ - 1m 41s
  6 412 tests ±0    5 891 ✔️ ±0  521 💤 ±0  0 ❌ ±0 
44 616 runs  ±0  43 898 ✔️ ±0  718 💤 ±0  0 ❌ ±0 

Results for commit c6ff065. ± Comparison against base commit 645f1ed.