sessionctx: Set foreign_key_checks = OFF

[morgo]

What problem does this PR solve?

Partially address #8349 by making metadata clearer that foreign keys are not supported.

What is changed and how it works?

This does two things:

1. Changes the default for foreign_key_checks to OFF (differs from MySQL default)
2. Shows a warning (and refuses to change) foreign_key_checks to ON. Since this is not supported by TiDB.

Check List

Tests

• Manual test (add detailed scripts or steps below)

I've manually verified it works. It does not actively change code, since it is currently an unused option - but I can add unit tests if desired.

Code changes

• Minimal

Side effects

• Breaking backward compatibility (for example, some exotic loaders may check for a warning on reload.)

Related changes

• Need to update the documentation (default differences from mysql)
• Need to update the tidb-ansible repository (I'm not sure - I don't think it is required).

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This change is 

[gregwebs]

LGTM

[ngaut]

LGTM, but defer to @shenli @winkyao

[shenli]

@morgo Please add a unit test case.

[ngaut]

/run-all-tests

[morgo]

/run-all-tests

[zz-jason]

LGTM