Bump UBI version to 9.8

Bumps the default UBI_TAG in base-images.sh from 9.7 to 9.8, as !76 (merged) and !73 (merged) did for 9.7 and 9.6.

Renovate moved BASE_TAG to ubi9-micro 9.8 on Repo1's gitlab-base, gitlab-kas and kubectl (for example gitlab-base!190). release-helper.sh writes BASE_TAG for those three from UBI_TAG, so the 19.5.0 release run moved them back to 9.7 (gitlab-base!194), and every run does the same until the default changes.

flowchart LR
  A["Renovate bumps ubi9-micro to 9.8<br/>on gitlab-base, gitlab-kas, kubectl"] --> B["release-helper.sh writes<br/>BASE_TAG from UBI_TAG"]
  B --> C{"UBI_TAG default"}
  C -->|"9.7 (today)"| D["BASE_TAG goes back to 9.7"]
  C -->|"9.8 (this MR)"| E["BASE_TAG stays at 9.8"]

At 9.8, UBI_TAG also moves the ubi9-minimal build stage and the ubi9-micro target stage on every project, matching CNG. Iron Bank publishes both at 9.8 (ubi9-minimal, ubi9-micro).

An AI coding assistant helped draft this change and its description.

Related issues

• Related to Update UBI images to v9.8 (CNG!2940 - merged)