Fix Vale issues for ./administration/feature_flags.md

The following issues occurred when we ran a linting tool called Vale against this topic. To fix the issues, open a merge request for the markdown file, which you'll find in the /doc directory. The first number on each line indicates the line number.

If you feel any of these warnings are incorrect, feel free to create an issue with the tw-test label so we can improve our Vale tests.

| 130:35:gitlab.FutureTense:Avoid using future tense: "will remove". Use present tense instead. |
| 131:27:gitlab.FutureTense:Avoid using future tense: "will no". Use present tense instead.     |
| 131:65:gitlab.FutureTense:Avoid using future tense: "will become". Use present tense instead. |