docs: remove Fantom and Moonbeam from supported chains matrix

[buddies2705]

Removes the Fantom and Moonbeam rows from the chain × product matrix on docs/blockchain/supported-chains.mdx.

Change

Two table rows deleted, nothing else:

-| Moonbeam | ✓ | — | — | ✓ | ✓ |
-| Fantom | ✓ | — | — | ✓ | ✓ |

Table integrity verified after the edit: 23 data rows remaining, uniform column count, no orphaned separators, no remaining references to either chain in the file.

Still inconsistent elsewhere — not changed here

These were left alone deliberately, since they are separate claims that need their own call:

File	What it says	Why it was left
docs/blockchain/introduction.md:51	Prose list still reads … Celo, **Moonbeam, Fantom**, Cronos, …	Directly contradicts this PR. Likely wants the same treatment — flagged for a follow-up.
docs/labels/address-labels-api.md:352,404	Fantom listed among label-supported EVM chains	Different product; labels coverage may genuinely still include Fantom. Needs a product decision, not a blind delete.
docs/graphql/dataset/network.md:34,39	fantom: 250, moonbeam: 1284	Chain-ID reference values, not support claims. Should stay unless the V1 network enum has actually dropped the identifiers.

The page's "V1 historically covers 40+ blockchains" note is unaffected — the matrix is explicitly a short list, not the full catalog.

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