feat: support mount sa token explicitly if automount is false (#6815)

[ti-chi-bot]

This is an automated cherry-pick of #6815

• If automountServiceAccountToken is set to false, br and discovery should mount sa token explicitly

[ti-chi-bot]

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[ti-chi-bot]

@ti-chi-bot: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-e2e-kind-tngm	95d8e2b	link	false	/test pull-e2e-kind-tngm
pull-e2e-kind-basic	95d8e2b	link	false	/test pull-e2e-kind-basic
pull-e2e-kind-dmcluster	95d8e2b	link	false	/test pull-e2e-kind-dmcluster
pull-e2e-kind-scale-simultaneously	95d8e2b	link	false	/test pull-e2e-kind-scale-simultaneously
pull-e2e-kind-across-kubernetes	95d8e2b	link	false	/test pull-e2e-kind-across-kubernetes
pull-e2e-kind-br	95d8e2b	link	false	/test pull-e2e-kind-br
pull-e2e-kind-serial	95d8e2b	link	false	/test pull-e2e-kind-serial
pull-e2e-kind-tidbcluster	95d8e2b	link	false	/test pull-e2e-kind-tidbcluster

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[liubog2008]

Prefer #6826