session: set tidb_server_memory_limit to defValue during bootstrap

[XuHuaiyu]

What problem does this PR solve?

Issue Number: close #41434

Problem Summary:

What is changed and how it works?

Set tidb_server_memory_limit to 80% if the user does not set it.

Check List

Tests

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

Side effects

N/A

Documentation

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

None

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• qw4990
• wshwsh12

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[XuHuaiyu]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 306fa5d

[XuHuaiyu]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: c7e324e

[XuHuaiyu]

/retest

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-6.5: #41497.