Rename `.htmltest.yaml` to `.htmltest.yml` #3004

Job	Run time
🔎 Check Event schema examples	14s
⚙️ Calculate baseURL for later jobs	1s
🔎 Check OpenAPI definitions examples	21s
🔎 Check JSON Schemas inline examples	9s
🔎 Validate OpenAPI specifications	17s
📢 Run towncrier for changelog	10s
🐍 Build OpenAPI definitions	23s
📖 Build the spec	33s
📖 Build the historical backup spec	0s
🔎 Validate generated HTML	6s
	2m 14s

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