README: remove TiDB Monthly

[CaitinChen]

What problem does this PR solve?

We'll deprecate the TiDB Monthly Update, so I remove the link.

What is changed and how it works?

What is changed and how it works?

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Code changes

• Has exported function/method change
• Has exported variable/fields change
• Has interface methods change

Side effects

• Possible performance regression
• Increased code complexity
• Breaking backward compatibility

Related changes

• Need to cherry-pick to the release branch
• Need to update the documentation
• Need to be included in the release note

[CaitinChen]

@lysu PTAL. Thanks.

[lysu]

/lgtm

[CaitinChen]

/merge

[ti-srebot]

@CaitinChen Oops! This PR requires at least 2 LGTMs to merge. The current number of LGTM is 1

[CaitinChen]

@tiancaiamao PTAL. Thanks.

[CaitinChen]

@crazycs520 PTAL. Thanks.