docs: fix d2:validatePattern escaping guidance

[karolinelien]

Related: https://dhis2.atlassian.net/browse/DHIS2-21359

Why

• Since expression-parser 1.4.2 (fix: Do not escape regex in d2:validatePattern [DHIS2-21359] expression-parser#100, shipped via rule-engine 3.8.1) the d2:validatePattern regex is used exactly as written.
• The docs still said the regex "needs to be escaped" and used \d examples. On current versions these no longer work: '\d{6}\/\d{2}\/\d' never matches a valid value, and '[^\d]*' no longer catches digits.
• Reported on CoP: https://community.dhis2.org/t/program-rule-expression-worked-previously-but-suddenly-failed-on-android-capture/76877

What

• Examples use single backslashes. Added a note on the versions where backslashes had to be doubled: core before 2.41.9 / 2.42.6 / 2.43.2, Android Capture before 3.4.2, Capture web app before 106.2.3.
• Corrected the example description from 9999/99/9 to 999999/99/9 to match \d{6}.
• Same change on the Maintenance and Metadata Management program pages.

How verified

• Version boundaries checked from the rule-engine version in dhis2-core pom.xml and capture-app yarn.lock at each tag. 2.43.2 is not released yet; it is taken from the patch/2.43.2 branch.
• New examples tested with Java and JS regex full-match.

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