Fix incorrect default for indexer

[adlerjohn]

Per celestiaorg/celestia-app#1178, the default indexer for app was changed to null from kv. However this was never reflected in

celestia-core/config/toml.go

Line 543 in 23f7a57

# 2) "kv" (default) - the simplest possible indexer, backed by key-value storage (defaults to levelDB; see DBBackend).

I'm not sure however if core still uses kv by default, and if this should be changed in this file. See celestiaorg/celestia-app#1178 (comment)

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Aside: actually the default was changed back to kv in celestiaorg/celestia-app#2039, but only in the branch v1.x, and not backported to main. main was then refactored in celestiaorg/celestia-app#2188, which still had null as the default.

[rootulp]

Seems like the proposal is to modify the default config.toml to:

# Options:
#   1) "null" (default)
#   2) "kv" - the simplest possible indexer, backed by key-value storage (defaults to levelDB; see DBBackend).
# 		- When "kv" is chosen "tx.height" and "tx.hash" will always be indexed.
#   3) "psql" - the indexer services backed by PostgreSQL.
# When "kv" or "psql" is chosen "tx.height" and "tx.hash" will always be indexed.
indexer = "null"

IMO the descriptions for those options could also be improved.