Fix for CS8714 add where T : notnull

[JohanLarsson]

    public static class PooledDictionary
    {
        /// <summary>
        /// Borrow a dictionary, dispose returns it.
        /// </summary>
        /// <typeparam name="TKey">The type of keys.</typeparam>
        /// <typeparam name="TValue">The type of values.</typeparam>
        /// <returns>A <see cref="PooledDictionary{TKey,TValue}"/>.</returns>
        public static PooledDictionary<TKey, TValue> Borrow<TKey, TValue>() => PooledDictionary<TKey, TValue>.Borrow();
    }

[JohanLarsson]

namespace N
{
    using System.Collections.Generic;

    public class C<TKey, TValue>
    {
        private readonly Dictionary<TKey, TValue> map = new Dictionary<TKey, TValue>();
    }
}