Fix FlexForm TCA deprecations

[jenni-prefa]

Problem

Several TCA deprecations in the FlexForm definition trigger E_USER_DEPRECATED errors in TYPO3 v12+. Example deprecation message:

/var/www/html/vendor/typo3/cms-core/Classes/Configuration/FlexForm/FlexFormTools.php:642
FlexFormTools did an on-the-fly migration of a flex form data structure. This is deprecated and will be removed. Merge the following changes into the flex form definition "cacheHeaderDuration":
The TCA field 'cacheHeaderDuration' in table 'dummyTable'" defines eval="int". The field has therefore been migrated to the TCA type 'number'. This includes corresponding migration of the "eval" list, as well as obsolete field configurations, such as "max". Please adjust your TCA accordingly.

Changes

1. Migrate cacheHeaderDuration from type=input + eval=int to type=number
2. Migrate secretKey from eval=password to type=password
3. Migrate customHost from type=input + renderType=inputLink to type=link
4. Migrate select items from indexed array keys to associative label/value keys

References

1. https://docs.typo3.org/c/typo3/cms-core/main/en-us/Changelog/12.0/Feature-97193-NewTCATypeNumber.html
2. https://docs.typo3.org/c/typo3/cms-core/main/en-us/Changelog/12.0/Feature-97104-NewTCATypePassword.html
3. https://docs.typo3.org/c/typo3/cms-core/main/en-us/Changelog/12.0/Feature-97159-NewTCATypeLink.html
4. https://docs.typo3.org/c/typo3/cms-core/main/en-us/Changelog/12.3/Deprecation-99739-IndexedArrayKeysForTCAItems.html