threshold of `gctuner` need to recalculate when `ServerMemoryLimit` is updated

[disksing]

Bug Report

1. Minimal reproduce step (Required)

update global variable tidb_server_memory_limit

2. What did you expect to see? (Required)

threshold of gctuner should be updated according to new value of memory limit

3. What did you see instead (Required)

threshold of gctuner is not updated (because it is only updated when we update tidb_gogc_tuner_threshold)

4. What is your TiDB version? (Required)

master