Spec

Fix schema of `m.receipt` EDU #1711

Job	Run time
⚙️ Calculate baseURL for later jobs	4s
🔎 Validate OpenAPI specifications	16s
📢 Run towncrier for changelog	6s
🔎 Check Event schema examples	14s
🐍 Build OpenAPI definitions	22s
📖 Build the spec	1m 1s
📖 Build the historical backup spec	0s
🔎 Validate generated HTML	13s
	2m 16s

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