Improve shields and header in README (Close #3017)

[3t8]

Tracking issue

#3017

Describe your changes

• Add links for commits, milestones, openssf and twitter
• Remove plastic style for consistency
• Remove newline in a tags to prevent unwanted underline on hover
• Bump milestone to current version
• Add link to LF AI & Data image

Check all the applicable boxes

• I updated the documentation accordingly.
• All new and existing tests passed.
• All commits are signed-off.

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[samhita-alla]

This formatting somehow doesn't look clean. Is the anchor tag on the new line the reason for underscores?

[3t8]

Yes, everything before the closing anchor tag is interpreted as text, including newlines. By default the style is underlined and we cannot change that on github.
In alternative we could put everything on the same line
<a href="https://github.com/flyteorg/flyte/commits"><img src="https://img.shields.io/github/commit-activity/w/flyteorg/flyte.svg" alt="Commit Activity" /></a>
but I thought it would make more sense to have the img tag on a new line even if the closing anchor doesn't have the same indentation as the opening one

[samhita-alla]

Can you try replacing anchor tags with https://stackoverflow.com/a/61072867/9695360 and check if this works?

[3t8]

We wouldn't be able to keep them centered if we go for markdown. The closest solution to mixing both is something similar to this https://stackoverflow.com/questions/29368902/how-can-i-wrap-my-markdown-in-an-html-div but the only difference would be using markdown image tags instead of html img tags. They all still have to stay on one line

[welcome]

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