Upgrade Go 1.25 → 1.26

[asparuhft]

Description

What

Automated upgrade of Go references from 1.25 to 1.26.

• go.mod: bumped only when it declared go 1.25
• Dockerfiles: golang:1.26 (preserving tag suffixes like -alpine)
• CI (GitHub Actions/CircleCI): matrices, go-version, and GO_VERSION vars updated (1.26)
• .go-version / .tool-versions: set to 1.26

A go mod tidy was run for modules that changed.

Why

Estate-wide alignment on Go 1.26 to match current platform/tooling and reduce drift.
Link to ticket: https://financialtimes.atlassian.net/browse/UPPSF-7052

Anything, in particular, you'd like to highlight to reviewers

• Please sanity check CI workflows if they pinned older images or used custom version matrices.
• Confirm any multi-module repos built as expected after the bump.

Scope and particulars of this PR (Please tick all that apply)

• Tech hygiene (dependency updating & other tech debt)
• Bug fix
• Feature
• Documentation
• Breaking change
• Minor change (e.g. fixing a typo, adding config)

DoD - Ensure all relevant tasks are completed before marking this PR as "Ready for review"

• Test coverage is not significantly decreased
• All PR checks have passed
• Changes are deployed on dev before asking for review
• Documentations remains up-to-date
• OpenAPI definition file is updated
• README file is updated
• Documentation is updated in upp-docs and upp-public-docs
• Architecture diagrams are updated

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