Attempts to fix Units.yml false positive for '1st'

What does this MR do?

In #387748, @sarahgerman caught a problem where the Units.yml test flagged an instance of 1st as being a unit-plus-number (1s). While low priority, let's see if we can fix it. My first idea is to add a word boundary character after the s in the regex.

The problem page identified in !108134 (merged) was doc/ci/pipelines/cicd_minutes.md, which has since moved to doc/ci/pipelines/compute_minutes.md.

# See the findings locally
$ vale --no-wrap --filter='.Name=="gitlab.Units"' doc/**/*.md

• Before: 126 findings
• After: 124 findings

When I restored the text in doc/ci/pipelines/compute_minutes.md, the previous false positives were NOT flagged. Also, these false positives were eliminated:

• doc/architecture/blueprints/ci_scale/index.md line 27
• doc/topics/cron/index.md line 45

Related issues

• Related to Updated 1st so it passes linting (!108134 - merged) which tried to find a way around the problem
• Closes Docs linting: Vale units rule returns a false p... (#387748)

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