Avocado should validate readme tag follow yyyy-mm-dd format.

[ruowan]

The severity should be warning. There is Microsoft API guideline rule. The api-version should follow this format. So we also need to validate readme tag to make sure the tag follow this format.

For example, https://github.com/Azure/azure-rest-api-specs/edit/main/specification/cognitiveservices/data-plane/Language/readme.md

For this readme, we should both validate the tag title and tag value.

The tag value of this readme is
[
'release_2022_10_01_preview',
'release_2022_07_01_preview',
'release_2022_05_15_preview',
'release_2022_05_01',
'release_2022_03_01_preview',
'release_2022_02_01_preview',
'release_2021_11_01_preview',
'release_2021_10_01',
'release_2021_07_15_preview',
'release_2021_05_01_preview'
]

Which is following our expected. should be like package-2022-10-01-preview