*: upgrade go version to v1.25.6 to avoid security vulnerabilities

[djshow832]

What problem does this PR solve?

Issue Number: close #1078

Problem Summary:

Target: `usr/bin/tiproxy` (gobinary) (7 vulnerabilities)
[MEDIUM] Risk Library:  golang.org/x/crypto  Installed: `v0.40.0`
[CVE-2025-47914](https://nvd.nist.gov/vuln/detail/CVE-2025-47914) - MEDIUM - Fixed: `0.45.0`
[CVE-2025-58181](https://nvd.nist.gov/vuln/detail/CVE-2025-58181) - MEDIUM - Fixed: `0.45.0`
[UNKNOWN] Risk Library:  stdlib  Installed: `v1.25.5`
[CVE-2025-61726](https://nvd.nist.gov/vuln/detail/CVE-2025-61726) - UNKNOWN - Fixed: `1.24.12, 1.25.6`
[CVE-2025-61728](https://nvd.nist.gov/vuln/detail/CVE-2025-61728) - UNKNOWN - Fixed: `1.24.12, 1.25.6`
[CVE-2025-61730](https://nvd.nist.gov/vuln/detail/CVE-2025-61730) - UNKNOWN - Fixed: `1.24.12, 1.25.6`

Target: `usr/bin/tiproxyctl` (gobinary) (3 vulnerabilities)
[UNKNOWN] Risk Library:  stdlib  Installed: `v1.25.5`
[CVE-2025-61726](https://nvd.nist.gov/vuln/detail/CVE-2025-61726) - UNKNOWN - Fixed: `1.24.12, 1.25.6`
[CVE-2025-61728](https://nvd.nist.gov/vuln/detail/CVE-2025-61728) - UNKNOWN - Fixed: `1.24.12, 1.25.6`
[CVE-2025-61730](https://nvd.nist.gov/vuln/detail/CVE-2025-61730) - UNKNOWN - Fixed: `1.24.12, 1.25.6`

What is changed and how it works:
Upgrade go to v1.25.6 and upgrade golang.org/x/crypto to latest.

Check List

Tests

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

Notable changes

• Has configuration change
• Has HTTP API interfaces change
• Has tiproxyctl change
• Other user behavior changes

Release note

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

None

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[wuhuizuo]

/test pull-check pull-unit-test pull-build

[djshow832]

/retest

[djshow832]

/retest

[djshow832]

refer to #1080