[CI] Decommission steps to test packages in stack 7.x

Remove the CI steps related to 7.17 stack version since it reached the end of support date of 15 January 2026, as stated in the [Elastic Product & Version End of Life Policy](https://www.elastic.co/support/eol).

- Integrations:
- Daily 7.x job: https://buildkite.com/elastic/integrations/builds/38972
- Configuration:
- https://github.com/elastic/integrations/blob/b0ed333a5339ce07f2f2da4ac925ac332f7d29ca/.buildkite/pipeline.schedule-daily.yml#L18-L29
- https://github.com/elastic/integrations/blob/b0ed333a5339ce07f2f2da4ac925ac332f7d29ca/catalog-info.yaml#L188
- Update references to 7.x in [docs about CI pipelines](https://github.com/elastic/integrations/blob/bb3dc2377d9bdc740131bceef51e3fb4c75bab35/docs/ci_pipelines.md)
- Update CI scripts logic for 7.x stacks:
- https://github.com/elastic/integrations/blob/bb3dc2377d9bdc740131bceef51e3fb4c75bab35/.buildkite/scripts/common.sh#L512-L516

Currently, there is just one package that is being tested in integrations repository for this stack version ([Buildkite build example](https://buildkite.com/elastic/integrations/builds/38972)): `bluecoat`

That `bluecoat` package is also tested in the 8.x stack daily builds. Example: https://buildkite.com/elastic/integrations/builds/38973

As a note, according to the title of this package, this package is deprecated:
https://github.com/elastic/integrations/blob/b0ed333a5339ce07f2f2da4ac925ac332f7d29ca/packages/bluecoat/manifest.yml#L3-L7

Relates https://github.com/elastic/elastic-package/issues/3325