What problem does this PR solve?

Issue Number: close #39150

Problem Summary:
The collate.newCollationEnabled variable is set to 1 when the collate package is initialized. When using this package, this value is not modified according to the config of the TiDB cluster (i.e., new_collations_enabled_on_first_bootstrap).

As a result, the user sets new_collations_enabled_on_first_bootstrap to false in the TiDB cluster, and it is still treated as true when br is executed.

What is changed and how it works?

Modify collate.newCollationEnabled according to the config of the cluster.

Check List

Tests

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

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Manual test

• create a cluster with tidb config new_collations_enabled_on_first_bootstrap: false.
• create a table with collation 'utf8mb4_0900_ai_ci': create table books (id bigint auto_increment,name varchar(64), primary key (id)) CHARSET utf8mb4 COLLATE utf8mb4_0900_ai_ci;
• insert several records to the table.
• execute a full backup
• create another cluster tidb config new_collations_enabled_on_first_bootstrap: false.
• restore the backup to the new cluster => success

Release note

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

Fix a bug that: "Error Unsupported collation when new collation is enabled: 'utf8mb4_0900_ai_ci'" even if new_collation_enabled is false.