Remove PDO from the codebase (still needs docs changes)

[GuySartorelli]

PDO has been deprecated since the acceptance of this RFC. See deprecation notice.

Remove the deprecated PDO code from postgres and update docs

Acceptance criteria

• The relevant PDO code is removed from silverstripe/postgresql
• The changelog notes the removal of PDO
• Any documentation that mentions PDO has been updated

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Already achieved acceptance criteria

• There is an appropriate deprecation notice in case someone tries to use PDO
• CMS 5 has no code related to PDO
• CMS 5 CI doesn't include a PDO run
• CMS 4 CI still includes a PDO run

Already merged PRs

• API Remove support for PDO #10499
• FIX Don't include PDO in matrix for CMS 5 gha-generate-matrix#33

PRs

• DOC Remove reference to PDO silverstripe-behat-extension#238
• DOC Remove references to PDO developer-docs#186
• MNT Remove references to PDO #10726
• API Remove PDO support silverstripe-postgresql#145
• API Remove PDO support creative-commoners/silverstripe-proxy-db#1

[GuySartorelli]

Reopening this - we still need to do the docs changes