Prepare 7.26.0

[wing328]

PR checklist

• Read the contribution guidelines.
• Run the following to build the project and update samples:

  ./mvnw clean package || exit
  ./bin/generate-samples.sh ./bin/configs/*.yaml || exit
  ./bin/utils/export_docs_generators.sh || exit
  

  (For Windows users, please run the script in WSL)
  Commit all changed files.
  This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master.
  These must match the expectations made by your contribution.
  You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*.
  IMPORTANT: Do NOT purge/delete any folders/files (e.g. tests) when regenerating the samples as manually written tests may be removed.
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.

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Summary by cubic

Prepares 7.26.0 by bumping project metadata to 7.26.0-SNAPSHOT, updating docs to point to 7.25.0 as the latest stable, and regenerating samples to reflect the new snapshot. This aligns master with the upcoming minor release and keeps install instructions stable-focused.

• Bumps versions to 7.26.0-SNAPSHOT in the root and module POMs and in plugin properties (Gradle, Mill, CLI, Online, Maven, Core).
• Updates docs: README and installation pages reference openapi-generator-cli 7.25.0; master header shows 7.26.0; plugin snippets use openapi-generator-maven-plugin, openapi-generator-gradle-plugin, and openapi-generator-mill-plugin 7.25.0.
• Regenerates samples; updates .openapi-generator/VERSION files and @generated comments from 7.25.0-SNAPSHOT to 7.26.0-SNAPSHOT; no functional changes to generated code.
• No generator logic changes or migrations required; CI should validate regenerated outputs.

^{Written for commit f250fd2. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 3000 files

<sub>Note: This PR contains a large number of files. cubic selects up to 200 of the highest-priority eligible files for this review, so some files may not have been reviewed.

Re-trigger cubic</sub>