Update lindah.toml for tesntet-15

[alexlindah]

Description

All previous genesis validators should name their PRs "Update {validator_alias}.toml for tesntet-15" and provide links to previous PRs merged.

#1430

If this is an UPDATE for a previous genesis validator

Checks are made against your net_address. If this has changed since the previous testnet, make sure you provide links of previous prs merged from your previous git username.

Thanks in advance!

Checklist before merging

• Only one toml is added in this PR
• The file being added is indeed a .toml file
• The toml being added is to the correct folder, and only to the correct folder
• The eth_hot_key and eth_cold_key are present
• The email, discord, elements telegram, and twitter fields are present and valid