variables: bump TiDB Operator to v1.6.6 and TiDB to v8.5.7 (#3164)

[ti-chi-bot]

This is an automated cherry-pick of #3164

What is changed, added, or deleted? (Required)

Bump the version variables on release-1.x for the TiDB Operator v1.6.6 release:

• tidb_operator_version: v1.6.5 -> v1.6.6
• tidb_version: v8.5.5 -> v8.5.7

This is the release-1.x counterpart of the v1.6.6 release. The release notes themselves are added on main (#3163) and cherry-picked to release-1.x; variables.json is bumped here directly because on main it tracks v2 (v2.0.0).

Which TiDB Operator version(s) do your changes apply to? (Required)

• main (the latest development version for v2.x)
• v2.0 (TiDB Operator 2.0 versions)
• release-1.x (the latest development version for v1.x)
• v1.6 (TiDB Operator 1.6 versions)
• v1.5 (TiDB Operator 1.5 versions)
• v1.4 (TiDB Operator 1.4 versions)
• v1.3 (TiDB Operator 1.3 versions)

What is the related PR or file link(s)?

• Companion release notes PR on main: en,zh: add v1.6.6 release notes #3163
• TiDB Operator v1.6.6 release commits on release-1.6: pingcap/tidb-operator@v1.6.5...release-1.6

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/approve

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