Enforce GraphQL descriptions styleguide for A/The

What does this MR do?

Our GraphQL description styleguide states:

Do not start descriptions with The.

Previously, @eread has also expressed that this should extend to 'A'.

This change adds a new offence to our Graphql/Descriptions cop for description strings that start with "The" or "A".

Existing offenses have been added to .rubocop_manual_todo and will be fixed in #337596 (closed).

How to setup and validate locally (strongly suggested)

Remove one of the lines added to .rubocop_manual_todo in this MR and then run rubocop on it. It will display the offense.

rubocop app/graphql ee/app/graphql

Does this MR meet the acceptance criteria?

Conformity

• [-] I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• [-] I have added/updated documentation, or it's not needed. (Is documentation required?)
• [-] I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• [-] I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• [-] I have tested this MR in all supported browsers, or it's not needed.
• [-] I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.