replace allowEncodedSlashes with enableStrictUrlValidation

[richard-salac]

Describe your pull request here:
Documents API ML PRs zowe/api-layer#4830, zowe/api-layer#4812 which removed apiml.service.allowEncodedSlashes in favor of apiml.security.enableStrictUrlValidation. It is a security related breaking change.

List the file(s) included in this PR:

• configuration-url-handling.md — reworked "Using encoded slashes" into "Using strict URL validation"; added a note that allowEncodedSlashes was removed in V3.5 (inverse meaning: set enableStrictUrlValidation: false to permit encoded
  slashes).
• zowe-yaml-configuration.md — replaced the allowEncodedSlashes property entry with enableStrictUrlValidation.
• troubleshoot-apiml-error-codes.md — removed the retired ZWEAG702E error code.
• configuration-routing.md / advanced-apiml-configuration.md — updated cross-links and sidebar label.

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